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That One Boss / Fate/Grand Order

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    Arc 1: Observer on Timeless Temple 
  • The Camelot chapter in its entirety is especially hard, particularly if you don't cheat and look up exactly how each boss's particular gimmick works, but several bosses stand out in their own level of difficulty.
    • Gawain charges up his NP twice as fast, making him faster than normal Assassins and Archers at charging his Noble Phantasm and he has a massive special resistance buff especially in the first two fightsnote , which will help ensure he survives long enough to use it. The saving graces are that you only have to deplete 25% of his HP bar in the first fight and his special resistance is weaker in subsequent fights.
    • Mordred starts with a full NP bar, and charges up to full again each turn. She doesn't use her Noble Phantasm in the first half of her first battle (she uses Extra attacks like a shadow servant instead), but during the second part of her first fight and last battle, Mordred will use Clarent Blood Arthur each turn unless you stun her or drain her NP bar, the latter, also with Gawain, not even being a full-on guarantee with them having a skill that can recharge their NP bar.
    • The Final Boss, Goddess Rhongomyniad, lacks the passives Gawain and Mordred had, but her primary kit alone is more than enough to make a fight as frustrating as the other two. Her NP is an party-wide attack which ignores invincibility and gives her a Charge afterwards, necessitating the use of Guts and/or Artoria Caster. She also has a skill that can further recharge it and remove her debuffs in the process, something she is NOT against spamming if she still has a turn left, making her use of it nearly unpredictable. Combined with her Charisma and Mana Burst, these kits will necessitate killing her as fast as possible, which is not easy with her 671k HP for her second and last battle.
  • Babylonia as a level is sometimes easier, but several fights take the difficulty up a few notches.
    • Quetzalcoatl comes with a buff that decreases your Servant's damage depending on their alignment. Good-Aligned servants do absolutely nothing, when Neutral and Evil Servants still only do half damage. There are a handful of Servants who aren't any of the three alignments (Bride, Insane, Summer), but at the time of the chapter's release there were only four in the game and two are Berserkers (Heracles and Berserker Lancelot), while the other two are Limited Servants (Nero Bride and Summer Tamamo)note . And the second battle not only gives her an additional defense buff, but also features the first instance of your dialogue choices actually mattering, with the top choice making her even stronger with a permanent Attack buff.
    • The Chaos Tide-corrupted Ushiwakamaru clones are very painful to fight in all battles where they are involved.
      • The first battle isn't so hard, despite charging her NP to full every turn, since she's a Berserker and her NP is single target, meaning that you can focus fire on her before she does too much damage. However, the second time you fight against SIX of her, and she still charges her NP each turn, meaning you need to have Servants with strong AOE NPs at the ready and/or plenty of Evasion, Invincible, or really strong defense buffs to survive.
      • The third and final battle against her removes her ability to charge her NP to full every turn, but in exchange, her class is changed to one that's neutral to every single class, and she gets the ability to spawn clones with full HP so long as there are less than three enemies and the original is alive. Her skill-set also makes her rather dangerous since it includes a dodge, NP charge, and a full-party attack buff. And to make things worse, your entire party lineup is shuffled around at the beginning of the battle and anyone who's active takes damage at the beginning of every turn.
  • Fittingly, the Final Boss of the first story arc is this in his second, and most-powerful form. He has a brutally-powerful area Noble Phantasm that reduces the effectiveness of a random card type by 100% the turn after he fires it. His basic attacks inflict three-turn debuffs to Critical Star generation, Attack, and Defense, and while the first two have very-high hitcounts to at least charge the NP gauge of whoever he's targeting, the third is single-hit, and his only basic skill gives him a free pip of Charge. He is immune to damage from Noble Phantasms and the extra damage from critical hits for the first three turns, and on the first turn of the battle, he'll wipe out the entire front line of an unprepared team by surprise-popping a one-time skill that instantly fills his Charge bar and grants him a titanic 300% NP damage buff once and Pierce Invulnerability. In short, a battle all-but designed to involve the use of a three-Command Seal revive unless you know how to prevent that skill (which is easy, but good luck trying to figure it out).

    Arc 1.5: Epic of Remnant 
  • Every single fight with Avenger of Shinjuku (Hessian Lobo), especially the last one. Not only do they constantly use buffs to increase their critical damage, defense, their attack and can clear all their debuffs every turn, every fight has a special gimmick. For the first and last fight, they'll inflict a defense debuff on one of your Servants every turn. The second fight has everyone taking 1000HP of damage at the start of each turn. Then for the third fight, they'll activate a one-hit dodge every turn. The last fight also has them inflict a very high attack debuff on one of your Servants. To make matters worse, their single-target NP has a sure-hit buff to go through any evasion and even if you have invincibility up, it has a high chance to inflict instant death. At least the first two fights have them faced as a Rider, so it's possible to use the class triangle against them, but for the final two fights they have the Avenger class, which means the only class available to deal super effective damage to them at release (Berserkers) have to contend with all of said dangers on top of their fragility, and the other class which can battle Avengers (Moon Cancer, which released later), is one of the rarest classes in the game, existing only in as the free Servant from an event for years and only as a limited 5* event Servant for years after that. It was three years after the chapter's release that a Moon Cancer entered the general pool, and even that one was a 5*.
  • One of the very first story battles in the Fate/Extra CCC crossover event is against BB with 400,000 HP, complete with BB Slots that will have a chance of giving you a 20% debuff in something. She hits really hard, can give herself critical buffs to make herself harder and she will constantly buff herself, only receives normal damage from standard classes and has only one weakness in Rulers (the class infamous for not having damage dealers), can clear all debuffs and grant herself debuff immunity, and her NP will likely kill any Servant unfortunate enough to receive it. All in all, she's a frustrating roadblock to the rest of the event.
  • Megalos is Heracles turned up a notch, as not only has he inherited self-buffs like Bravery and Mind's Eye (False), the Berserker class, and an un-nerfed God Hand (as twelve separate unremovable Guts buffs, which you thankfully only have to deplete four at a time over the course of several battles), he's also got a few new tricks up his sleeve, like a taunt every third turn which causes him to take a special multi-hit attack that will decimate the HP of the targeted Servant the turn after applying the taunt (and he will take a fourth action to activate the taunt, and do so even while stunned or charmed, while removing the taunt still causes him to take the special attack on a random target) plus a charged AoE attack, which also can be a critical hit, leading to many parties getting wiped out.
  • The hardest fight in the pseudo-singularity is probably the Dual Boss encounter with Megalos and Christopher Columbus, where, unlike prior encounters, both enemies have Break bars rather than Guts, one of them is a powerful Rider enemy to punish support Caster allies, and cracking the Rider's Break bar causes him to either grant Megalos Invulnerability Pierce for three turns, or to gain it himself if Megalos is gone. And while Megalos does gain three-turn vulnerabilities upon Break, first for Quick and then for Arts, he also gains Critical Damage and Rate buffs for the same, respectively, and is much more likely to spam his Mind's Eye (False) and gain Evade while at low health than in previous encounters. Also, to add insult to injury, the Rider ally has a skill that can give himself a free point of Charge gauge, de-synchronizing his Noble Phantasm turns from Megalos's. A winning team needs to be carefully constructed and able to deal with deadly, shifting battlefield conditions at a moments' notice.
  • The final boss fight with Caster of Nightless City (Scheherazade) can be incredibly annoying if you aren't prepared. She fields several waves of minions that take the field one after another, she has a charm skill which almost always works on male servants and buffs her defense, making her incredibly hard to kill. And if you crack one of her Break bars while her minions are still on the battlefield, she grants all of them the Taunt status for 10 turns, forcing you to defeat them first while she charms and attacks you with impunity. That's also not getting into her incredibly hard hitting AoE Noble Phantasm which is practically a One-Hit Kill against any servant with the King or Queen trait. Oh, and she takes greatly reduced damage from all servants with the aforementioned trait, so you can't just bring someone like Ozymandias or Quetzalcoatl and expect an easy win.
  • Almost every major battle in Shimosa can potentially count for newer players with underdeveloped rosters, as these battles force the player to use the (initially very mediocre) Support Musashi rather than their friends list and to put her in the front row. The Singularity's special bonus also encourages players to use Sabers, Archers, and Lancers for doubled Bond points, which can make these boss fights even harder. On top of both of these, each battle comes with gimmicks that make it harder, and some of them are pretty crazy:
    • Assassin Paradiso can serve as a massive roadblock many players without a strong offense-focused single-target Caster, as her normal attacks inflict attack down to your teammates and when her first health bar is broken, she can inflict a 1-turn stun to the party member whom attacked her last at the end of each of her turns for five turns. And this doesn't factor the other annoying skills she has (such as a three-turn Arts buff that also causes her to inflict Curse with her normal attacks for the same duration, her Evasion skill that charges her already-short Assassin Charge bar, and an NP seal skill that interferes with many Casters' game plan). Without debuff cleanses or immunities, she's hard to outlast or stall, and without strong single-target damage (which is rare among Casters), she's hard to burst down. At least buff removal still counteracts many of her strategies.
    • The Dual Boss battle against Samghata Berserker and Kalasutra Rider takes the cake as both Servants use AOE NPs that can wipe out the party if taken unprepared and Berserker spams her defense down with charm skill a lot. They also significantly cripple the party merely by attacking as Berserker inflicts a debuff that drains the NP gauge of whomever they've attacked at the end of their turn, and Rider removes a fixed number of crit stars per attack. It gets even worse when Berserker's first bar is taken down as she not only gains permanent debuff immunity, but lowers debuff resistance of the party, which effectively ensures that every time she uses her charm skill everyone being affected and thus wasting a turn is a very real possibility. Rider meanwhile gets a crit damage up buff after her first bar breaks that, when combined with her crit chance up skill and various damage skills, can slaughter even an Assassin if RNG decides to hate you. The only saving grace is that their AOE NPs take a long time to charge thanks to their classes and they have no charge skills to speed them along.
    • The battle against Empireo Saber is equally difficult with his ability to remove buffs with any attack, inflicting multiple massive attack down debuffs that in this fight also counts as a NP charge skill, his charm immunity and his massive special defense buff (which can only be bypassed with NP damage) after taking out his 2nd HP bar. The only saving graces here are that with that defense buff comes an NP gain buff on your party, which is the game's way of screaming at you to burst him down with NPs, and that, as a Saber, he is vulnerable to being bursted down by single-target Archers with full Bond bonus, unlike the other frustrating bosses of the chapter. He also has a single-target Noble Phantasm, so at least you'll only lose teammates one at a time.
  • The closing boss of Epic of Remnant, the Foreigner Abigail Williams, is very mean. She gets a few introductory fights where you see that she has a timeloop mechanic, turning back the clock and regaining all her HP each time it's depleted, preventing you from just melting her with an NP. When the actual boss fight starts however, she also has an NP gauge, and she has a fairly meaty HP pool of roughly a million HP divided into 250k-ish per loop. Worse yet, she has all three of her Servant Skills which increases her NP charge (30% chance) and NP power, inflicts Terror and Defense Down on the whole party (used at the start of the fight, too!), and inflicts Attack Down and an NP drop on an enemy respectively. And her NP in question? A single-target that strips all defensive AND offensive buffs before impact. This NP is practically guaranteed death to anything that isn't an Alter Ego stacked on HP, and even if the target survives, all of their momentum will be gone. Abigail will get her NP off at least once unless you're seriously lucky with Skill rolls, and if the unfortunate target was your primary damage dealer, you'll be in for a rough ride as the boss can easily survive long enough to keep charging her NP and pick your party members off one by one.

    Arc 2: Cosmos in the Lostbelt 
  • Although the writing and atmosphere are significantly Lighter and Softer than the previous two chapters, the third Lostbelt is commonly considered to be one of the hardest chapters by far, bursting from the seams with all sorts of difficult battles:
    • Whether he's fought alone or alongside another Servant, Xiang Yu is a constant thorn in many a player's side in almost every battle he's in (apart from the final time he's fought); massive regular and critical damage output that is almost guaranteed to one-shot anything that isn't a Foreigner (and even that won't save a Foreigner from a critical shot), boasts an Evade skill that allows him to eat attacks for a turn, multiple HP bars to break through with their own effects, and his Noble Phantasm will completely nuke your party if you don't prepare for it beforehand. Also, since he is technically not a Servant, any skills or properties which only apply on a Servant (such as Jeanne's stun or bonus damage from Gilgamesh's NP) won't work on him. And on top of that, he appears as early as Verse 6, and once he starts appearing, he will be the most common named enemy you'll fight until he's finally killed for good. The first few fights at least only require him losing his first HP bar to achieve the win condition (in fact, his first Dual Boss fight with Prince of Lan Ling is quite merciful thanks to this), but the final section in Verse 9 and the first section in Verse 16 is when he has to be fought to the finish (which also happen to be his second Dual Boss fight with Prince of Lan Ling and his Dual Boss fight with Yui Mei-ren respectively, both of which are the most difficult fights against him in general as well).
    • Coming off an already grueling battle against a horde of Terracotta Warriors and Royal Guards, you would think that Old Li Shuwen would be a welcome respite from the brutal difficulty of the chapter; he's actually regarded as the most difficult fight of the entire Lostbelt. His abilities (which he can recast even if they're removed) include a triple attack buff (Though they’re a mere 1% boost, meant to fake you out) and a free NP charge as his health drops, a skill that allows him to negate class advantages and disadvantages, and boasts a heavy-hitting single target Noble Phantasm with a high instant death which he buffs with an invincibility pierce before firing it off, resulting in one hell of a Beef Gate so close to the eleventh hour of the chapter.
      • Likewise, people who have managed to beat Li Shuwen easily, either due to having a very meta setup, lots of preparation, or just pure skill, will be more likely to say that Han Xin's Zerg Rush the hardest part of the Lostbelt instead. Even with an Invulnerability Pierce CE on your main DPS, it's a long and grueling battle that won't be over very quickly since at least half of the enemies have Guts on top of their three-hit Evade, and even dedicated stall strategies will have a hard time due to each of the Mooks being able to Noble Phantasm very quickly.
    • The Lostbelt's resident king, Emperor Qin Shi Huang, isn't much better. Their permanent buff and NP can boost their critical rate and attack strength to such ridiculous levels that one-shotting an Avenger - a class that a Ruler has an explicit disadvantage against - isn't out of their capability. Their NP also boasts a one-turn invincibility buff, and their skills include abilities to remove all debuffs cast on them, another that can stun all three of your frontline Servants if you're unlucky and automatically maxes out their NP gauge each time you empty one of their health bars. The cherry on top of the sundae is also the little fact that they're a Ruler-class Servant - meaning that most of your Servants will be doing less damage by default unless you have some good Avengers and Berserkers on hand (which, as stated above, is only a small comfort given what they can do). Couple that with a permanent buff that restores ten thousand health per turn (along with a debuff immunity for the first bar and a removal of all party buffs for the second) and you've got a boss that even meticulously prepared teams will struggle to defeat. The only saving grace is that your party has a permanent Buster/Arts/Quick buff in the interest of ensuring your attacks have that extra damage needed to power through, but they will certainly be making you fight for every scrap of damage.
  • Lostbelt No. 4: Yuga Kshetra:
    • The final battle against Arjuna Alter deserves special mention here. Starting off, he has complete debuff immunity along with a passive damage resistance and a hefty attack up. His class being a Berserker means his attacks will really hurt, and with his buffs can overpower Foreigner resistance through sheer damage alone. Upon 1st Break, he buffs himself with even more attack power and a crit rate up. Even worse, upon his 2nd break, he takes a page out of Goetia's book to grant himself an NP damage increase, invincibility pierce and charge his NP gauge to the max (which means Total Party Kill without Guts or Caster Altria). You also can't bring friend supports here since it is locked on a Super Mode Karna NPC; while this makes the battle tricky like the Shimosa duels, at least this time Karna is much more useful and powerful than Shimosa Musashi as he's Level 100 at NP5 (and his NP deals bonus damage to Arjuna Alter) with maxed out skills alongside several bonus buffs such as Damage Up, 2000 HP heal every turn, and a one-time Guts, so building a team around supporting him and letting him single-handedly slaughter Arjuna for you is a very viable strategy. However, Karna's first skill is basically useless on Arjuna Alter due to the skill only inflicting debuffs on the enemy and God Arjuna having Debuff Immunity.
    • Even the battle beforehand is not much better. The first wave has the return of Limbo from Shimosa, who immediately drains your NP gauges by 50%. He is also an Alter Ego, whose AoE attacks can effortlessly whittle down your support servants and any Foreigners you brought along. Then you have to fight Arjuna Alter, who starts out by immediately charging his NP gauge. If you survive that and break his first bar, he inflicts a heavy Defense debuff on your party as well as removing their buffs. If his Anti-Evil skill is active or his NP is ready, Total Party Kill is possible.
  • Camelot, Shimosa, and SIN may have been serious contenders for "hardest Singularity/Pseudo-Singularity/Lostbelt" before its release, but Olympus has happily taken that throne for itself. In addition to having one of the highest difficulty thresholds across the entirety of the chapter and being a grueling Marathon Level to boot, the many, many boss fights have proven to be an impenetrable march of unforgiving face punches that will burn through your Command Spells and Saint Quartz like a wildfire in the nucleus of the sun before you knock the resident Tree of Emptiness down. Don't believe us? Consider the following:
    • The first major roadblock you'll face is the fight against Talos and Dioscuri. Talos is a Berserker whose basic attacks hit the entire team, on top of having a passive guts, and when you deplete its Break bar it'll inflict burn on your Servant whenever being attacked and attacking Talos (which means massive stack of burn damage). Dioscuri's Noble Phantasm pierces invincibility, and their skill set ensures that they can spam it as often as possible. Considering this fight happens as early as Chapter 3 (a point where most players will still be trying to feel their oats to figure out how to play through the Lostbelt), the player's first instinct upon getting ripped apart by the one-two punch of an AoE Beserker and a Servant battle all too reminiscent of the Mordred fight in Camelot is gonna be to lean your head down between your legs and kiss your ass goodbye.
    • Demeter is the second major roadblock you'll face in Olympus. She has the ability to stack multiple defense buffs (nullifying potential damage to basically zero for a turn), her critical attacks are basically a One-Hit Kill if unresisted, she can give herself 50k HP heal/debuff clear/debuff immunity/20k HP regen all in one skill, and she can fill three of her NP bars instantly. To top it all off, her first break bar removes all buffs from the party and inflicts 3-turn 3-time buff blocks (if you don't have buff removal resistance and debuff immunity up, both ultra-rare skills at low rarities, you're FUCKED), and breaking her for the second time grants her an unremovable 150K-HP Guts, dragging out the fight further and making it even more hair-pullingly frustrating. Her Noble Phantasm is also downright horrific, as it applies a huge Max HP Down debuff BEFORE she deals massive damage, nearly ensuring a Total Party Kill every time she uses it (and with her skill that fills three charges, you will see her use it a lot). Worse yet, you're forced to use the support Caenis the story gives you, and while decent she is much less useful then a friend's support caster. The only saving graces for this fight is that Demeter averts A.I. Roulette, using her skills with a set pattern so you'll know what fist she's about to hit you with as well as how her third break bar grants herself a Confusion debuff which gives a chance to inflict Skill Seal on herself (therefore, making it possible for her to be unable to cast one of her annoying skills for one turn), but all of these traits together make her the biggest Beef Gate of a boss at the time of the Lostbelt's release, and more than a few players have expressed intense frustration at just how hard this fight is. It's quite telling that not even Zeus, the RESIDENT LOSTBELT KING is considered as difficult as her.
    • Beast of Taming Tamamo Vitch Koyansakaya. It's fooled many a player into first believing it's a Hopeless Boss Fight, but you actually have to beat it to proceed. Its Nega-Weapon passive makes team composition very awkward, since its strengths and weaknesses are based on traits, being strong against non-Caster Hominidae servants (which is not the same as Humanoid, so good luck finding out who isn't weak to her), inflicting neutral damage on Casters, and weak only to the Tamamo Servants or Demonic Beasts (animalistic Servants like Avenger of Shinjuku or Asterios), with full type advantage basically restricted to Caster Tamamo. She also shrugs off critical and NP damage at the start of the battle for three turns, limiting a player's ability to burst her down and limit the pain, and her active skills are a headache and a half, including a three-turn chance to stun a character that heals her for 50,000 HP if the enemy is killed with it active, a 5-time, two turn Evade, and a 10-turn curse effect that would be less painful if her Noble Phantasm didn't apply curse-amplification. And while her second break actually helps the party (reducing her gauge and defense while applying a heal, cleanse, and 20% meter), the other two are typically painful, the first break instantly giving her max Charge and a crit damage buff while plopping DEF and Crit Generation Down on the party and her third granting three turns of boosted damage, five turns of good regeneration, and one hit of 150,000 HP Guts. Often compared to the infamous fight with Quetzalcoatl in Babylonia, except that fight wasn't anywhere near the battle of attrition that the Beast of Affection is.
    • The Dual Boss fight against Musashi and Sherlock, for one very big reason: Permanent Defensive Buff Nullification. That means no Defense stacking and no evasion or invincibility. It doesn't help that you're forced to use Caligula - a two-star Berserker Servant whose lack of viability has been a joke since the game's launch - as your support, meaning that if you want to try and lessen the sting of not being able to protect your Servants from the high damage output without heavy dedicated ATK debuffers, you're shit outta luck. Oh and for a final layer of frustration, Mash has to be put on the frontline, meaning the player can only slot in one character at first. (Not that a Friend Support short of Nightingale would help much anyway).
    • The final match against Dioscuri is pretty much a Luck-Based Mission and can be one absolute hell if you're unlucky. The fight itself is pretty straightforward until their first Break bar, at which point they buff themselves with an increased crit chance and the ability to gain a point of Charge every time they crit. This means it's entirely possible for them to kill two Servants with critical hits before unleashing their NP on the third (which you do not want, as above), all in a single turn. They're also immune to stuns and charms, so stalling won't necessarily work either. If you don't have enough taunting Servants and heavy hitters to finish the fight as fast as possible, Total Party Kill is certainly possible.
    • Your rematch with Wodime. This time you do have to defeat him, but he sure doesn't make it any easier. He has a variety of buffs on him (among Debuff immunity, Crit Resistance and others, he even heals himself when you land a Noble Phantasm on him) as well as complete neutrality to damage, meaning there's no real weakness to exploit. (Riders still resist though.) The buffs wear out over time, but refresh upon every break. While the fight itself is somewhat manageable, his first attack is a total dick move: He essentially pulls a Goetia and unleashes a supercharged NP on the first turn that cannot be blocked, meaning more likely than not you're going to have to rely on your backline to win unless you want to use that three-Command Seal revive.
  • Heian-kyō:
    • The Dual Boss fight against demonized Suzuka and Touta is a hair-puller for one specific reason: their massive damage resistance only wear out after they use their NP, and for only one turn. Problem is that they're Saber and Archer respectively, meaning they have different paces for NP attacks, and it only becomes worse: breaking Suzuka will allow both of them gaining one extra NP charge each turn (while she can also gain NP charges by her skill), and continuous NP spam from them will only end in one way unless you can grasp the little timeframe and finish them off.
    • Ibuki Douji may become unbeatable if the player doesn't break her twice fast enough. During her first two Break Bars she'll refill one-time Invincibility for each type of cards and Critical/NP Attacks every turn (of which they stack), and they must be cleared as much as possible before breaking her second Break Bar, because each Invincibility will boost her HP value up to seven digits, and there seems to have no upper limit for that, so it's entirely possible to fight her with over 2 million HP. And she's not just a Damage-Sponge Boss either; she boasts an Invincibility-piercing party-wide NP (and will immediately use it after you break her twice after absorbing your servants' NP), while using Skill Bind and highly reducing your damage for a turn on the first and every 5 turns. She will also randomly take away your Critical stars, stunning your Servants sans one or inflicting Curse, and her normal attacks are strong enough to deal massive damage. Talk about a slug fest against time. There is, however, the alternative of waiting for over 5 turns, as her invincibilities stacks won’t be used without her 2nd HP bar break, on top of preventing her from draining your party’s NP and any invincibilities she may have left after the 2nd break will increase her HP less so, yet not only will she still get a permanent Critical Chance increase, you’ll still have to deal with her rampant debuffs and Pierce Invincibility AOE NP, basically meaning either having to Break her as soon as possible with Pierce Invincibility of your own being mandatory or trying to turtle through, all while dealing with a plethora of painful debuffs, a hard to avoid NP and ridiculously high HP.
  • Avalon le Fae starts off slightly simpler than Olympus, but quickly starts to overshadow it, and shows that many of Chaldea's enemies within that chapter will not stand down without a fight:
    • Fae Knight Gawain has a recurring gimmick in which every turn, she will inflict a taunt buff on a random party member, forcing them to endure her attacks while she gets unremovable buffs that makes all her attacks work like a Berserker (though she still retains her weakness to Archers and other Berserkers) and Damage Reduction as well. The first battle against her is annoying because of how she depletes your Command Seals, though since it's meant to be a Hopeless Boss Fight, you can try to tank her attacks for seven turns, though this will leave you without any of your command seals if you do so. The second fight against her has you actually try to defeat her, though thankfully her Damage Reduction is removed by Muramasa after the third turn, and since you only have to deplete a single HP bar, you only have to endure a few turns before delivering a brutal counterattack. The third time, this time, you will have to defeat her with not just the Damage Reduction, but also with two health bars and forced story supports in the frontline! Though to compensate, Oberon gives out a unremovable defense debuff on Fae Gawain and he happens to be a decent Support Party Member, and she happens to gain the Threat To Humanity trait, which happens to be Altria Caster's main target since her third skill grants increased damaged towards those with that trait, but even then, without a hard-hitting ST Arts Archer/Berserker like Summer Altria or Vlad III and a Caster Tamano, this fight can really turn into a frantic rush to defeat each other. The fourth battle with her also has her consume the player's Command Seal, though this time, she's without her Damage Reduction, and she doesn't give out a taunt buff to a party member, essentially making it easier to break her two health bars.
      • Her final fight against you as Demon Hound Barghest is much harder than the rest because of one reason: Forced Story supports. You essentially have to get at least one Story Support in the frontline because you're reduced to five party members, and while both Gawain and Lancelot start off with max skills and NP and their Balance Buff, and Lancelot for similar reasons, unless you reduce her HP in time, you can expect your party to go bye-bye within seconds because she can remove a number of your buffs at regular basis, and upon her first HP bar break she'll remove all your buffs unless you counter it with Buff Removal Block, which is a rarity among Servants, and upon her second HP bar break, Barghest inflicts a permanent Damage Down buff (along with NP reduction and charge up her NP bar), though this is a nuisance at best. On the other hand, Demon Hound Barghest uses her skills in fixed order like Demeter so players can prepare for what's coming up, and all three forced members have fixed and useful skills to help with the fight, but it's still considerably harder than every fight before in this Lostbelt, especially Albion's Remnant (and likely because you faced it before Barghest), and it's just the beginning of something even worse.
    • The rematch against Fairy Knight Tristan is this for all the wrong reasons. The good news? You have a maxed out Altria Caster as a Story Support, and she's also wielding a Craft Essence that increases her NP Generation and Starting NP. The bad news is that she's your only party member. Keep in mind that she's basically geared towards support and not as a damage dealer, all while the boss whittles you down with her heavily damaging multi-hit attacks and Curse debuff stacking. Oh and top of this, Beryl has the complete audacity of summoning an Efreet after you've broken the boss' break bar. Fortunately, the last match against her this time has you bring your own party aside from some forced Story Servants which are really beneficial, though her curse stacking still makes her annoying to deal with.
    • The final battle against Fairy Knight Lancelot (not including her Albion form). Imagine Mordred's gimmick in Camelot, except with break bars, a single target NP, Forced story servants & no master skills or command seals. Upon 1st break, she applies a sure hit to herself to hit through evasion. Upon 2nd break, she also applies Invulnerability to herself for 1 turn. All this, on top of dealing with her onslaught of subsequently stronger Noble Phantasms and almost guaranteed crits, devolves the fight into a Damage Race blitz at the eleventh hour. The only upside in your favor is her typing as a Lancer, and the fact she has the Demonic Beast and Dragon traits, meaning Sabers like Rama and Sigurd can greatly help, but you still have to content with her ability to practically one-shot you every turn.
    • The Lostbelt Queen Morgan proves why she is considered the strongest of the Lostbelt Kings. Despite being a Berserker, she takes neutral damage from all classes while still dishing out effective damage to anything that isn't a Shielder or a Foreigner. She also has a passive which removes the two latest debuffs she has at the start of her turn, making any attack debuffs and stuns useless against her. Upon 1st break, she gains a permanent crit damage and crit rate up, along with a new passive that debuffs your attack and star generation at the end of each turn. But it gets even worse with her 2nd break, where ALL of her attacks removes ALL your buffs, her NP gauge charges thrice as fast and she gets a crit rate up & attack up buff at the end of each turn as well, of which it becomes a race against time to beat her before she levels your party flat.
    • Cernunnos makes previously insanely difficult bosses like the aforementioned Queen Morgan, Demeter, and Wodime look only relatively challenging, combining all the worst gimmicks you saw in the Lostbelt: damage reduction, Curse spam, Crit Rate boosting, and a Berserker class. The main gimmick is his constantly-stacking Curse Layer buffs which grant him stacking damage reduction for each one; he starts the fight with seven stacks, gains one every time his turn starts, and can gain one at random with every attack he makes. The only way to remove the Curse Layer stacks is by attacking him, which does three horrible things: heals him for 10k after the attack, hits the whole frontline with a Curse debuff, and hits the attacker with an 1k HP Drain — and these debuffs stack. If you don't have a ridiculously high debuff resistance/immunity (like First Hassan's Protection of Faith A+++ skill), and cleansing them on a regular basis, these debuffs combined with his regular Berserker attacks (which are all AoE, by the way) will chew through your Servant's HP like candy. His Break Bar gimmicks don't help: the first Break puts NP Seal and Skill Seal onto the frontline for one turn (and they're non-removable), the second Break increases his Crit Rate and gives him five more Curse Layer stacks, and on the third he will gain seven more Curse Layer stacks and puts a 10-turn debuff on your frontline that drains 5% NP every turn (which also isn't removable). Cernunnos is considered to be so difficult not even Oberon Vortigern, the actual Final Boss of the Lostbelt, is as difficult as he is, mostly because you live or die on the RNG praying that Cernunnos doesn't just attack endlessly and build up Curse Layer stacks faster than you can remove them. You are transparently encouraged to use Forced Story Altria Caster in this fight for her NP and her third skill, and it says something that the game allows you to use double Altria Caster here when duplicates of her aren't allowed anywhere else in the Lostbelt, almost as if the devs knew you might need a backup Altria Caster if Cernunnos decides to roll a Yahtzee and take her from 100% to dead in one turn. Also tellingly, this boss didn't come back for the the LB 6 Recollection Quest despite how iconic he was, simple because his fight was already hard enough.
  • Traum:
    • The duel against Bradamante and her mob forces you to use at least 4 story support Servants in the front, with only room for two extra Servants, which wouldn't be as hard, except she's being backed by Pope Joan, who proceeds to immediately grant her and the other generic servants Invulnerability and Debuff Immunity (even for those in reserve), forcing players to use Sherlock Holmes's NP to pierce the invincibility (and she also casts debuffs and stun one of your men in the two following turns). While the player could normally use the plugsuit to switch out one servant, in this fight the player is stuck with a chapter-exclusive Mystic Code that has great abilities but absurdly high cooldown even by Mystic Code standards and without switching ability, forcing the player to rush after Bradamante before she unleashes her NP (breaking her HP bar grants her Invincibility Pierce by the way), and even then the player's party might end up too battered to finish off the other servants.
    • The two Grand Battles against Kriemhild aren't as hard as those Berserker bosses above, but she's not a pushover either, for you're forced to use Siegfield as your support (while Kriemhild doesn't have Dragon trait), greatly reducing your choice of strategy; and Kriemhild's NP strips away all defensive buffs before attacking, which will spell certain death to her target. The first round involves taking out about 900K HP under a passive damage reduction, which can be a challenge in itself; during the second round she has a passive ability to grant her a charge after each round (so she fires her NP every 3 turn), and after you break her twice she'll immediately cleanse all her debuffs and unleash her NP on one poor sod, THEN she'll have max NP charge for the next 3 turns, resulting in a bloody carnage on your party if there aren't enough taunting decoy and/or Guts. To top it off, Kriemhild can also gain one NP charge from her skills, and she can force-taunt one of your servants to make sure the target is dead in a round or two. Do not underestimate this avenging woman.
  • As the final Lostbelt Chaldea has to face, Nahui Mictlan contains some understandably hard boss fights:
    • The very first fight in the Lostbelt pretty much establishes the difficulty, as you face the Foreign God U-Olga Marie. While two bars are required to be broken to end the fight, she will make your life hell if she isn't dealt with in time. First, she has Permanent Debuff Immunity, meaning that she cannot be debuffed in any way. Second, all of her skills in her kit are pretty designed to make your life hell: Ultra-Manifesto EX increases her chance to crit which is usually a death sentence to any party members she crits on while also sealing the entire party's NP and preventing them from healing for a single turn, Atomic Plant EX gives everyone a Burn Debuff of 10,000 damaged per turn which is dangerous if combined with her first skill while also instantly filling her NP bar with the small price of some of her HP, and from there, she either has a chance to use ULTIMATE-U C, which uses up her entire NP bar to grant her Ignore Invincibility, Buster, Crit Damage, and additional NP bars, or she can use her Extra attack, which deals AOE damage. Much like the Beast of Taming, she gains a tick into her NP Gauge upon defeating a party member, though thankfully she gets debuffed with an NP damage down Debuff in return.
    • Crimson Vermillion Beni-Enma is a dice roll if you don't have any Servants that resist or can nullify Instant Death or provide Guts, as every single attack she does has a chance to instantly kill, and at a terrifyingly high rate. Additionally, on her first Bar Break, she'll instantly charge her NP and Ignore any defense buffs, which is seriously bad news to players who don't have any Servants that provides party-wide Instant-Death Immunity/Guts. Upon her final Bar Break, she'll additionally gain an defense up buff against Chaotic and Evil servants, forcing players to run Servants that don't have those traits. The only saving graces is that if you run U-Olga Marie as a support servant and Instant Death is triggered on her, she'll gain a Guts buff that allows her to survive it, and First Hassan is available on your party alongside providing a party-wide Instant-Death Resistance buff, but he's locked to the final slot of your party.
    • Tlaloc is, for all intents and purposes, the final boss of Part I of Nahui Mictlan as the next fight against ORT is a Hopeless Boss Fight, but she can bring the pain regardless. Her first proper boss fight can be annoying if the player doesn't organize her gimmick, as she deals an unremovable Stun Debuff to any Servants that aren't floating or flying, and the criteria for it is extremely inconsistent, as some servants who have wings to fly are still affected by the Debuff, and all of the meta supports cannot avoid the Debuff. The only way to prevent it is to use Nemo as a Support servant, but Tlaloc knows about it, and will target him, turning the whole fight into an Escort Mission. She also has an passive HP Regen of 30,000 HP per turn, and as she's a Ruler, running an servant that deals massive damage while also providing class advantage such as Summer Erice is a must, while also being forced to deal with a reduced party size and Mash Kyrielight locked in the front row. Finally, upon her Bar Break, she inflicts a delayed Debuff that has a chance of stunning Servants while also dealing additional damage to them. Once you get past those, however, her second fight has her completely break loose and turn into a Berserker named Huitzilopochtli, which has her gain Invincibility for 3 attacks, a Defense Up buff, alongside another Invincibility buff for 3 attacks per turn, and she becomes immune to any debuffs that prevent her from taking a turn, and every 2 turns, she will fully charge her NP gauge alongside a one-turn NP damage buff, and while she does user her Extra attack instead, it's a AOE attack, and because she's a Berserker, she will hit hard on anyone that's not an Foreigner. Thankfully, you only need to break one of her bars, but good luck trying to do that in the first place with her massive defenses!

    Interludes/Rank Up Quests 
  • Jekyll's interlude. You have to fight two Jekylls, and one of them has more than 200,000 HP. What makes him especially hard is his Noble Phantasm, which both heals him and increases his HP maximum. Getting him to very low health and failing to finish him off before he uses it turns him into a Marathon Boss unless the player brought buff removal.
  • Babbage's NP upgrade interlude. It is a boss fight against Rider Fafnir with more than 200,000 HP, what's worse is you cannot bring any supports besides the type-disadvantageous Babbage. If you don't have any strong Assassin units like Jack, good luck.
  • Robespierre's ghost from d'Eon's Interlude. It has over 900,000 hitpoints and will constantly stun your Servants and drain their gauges with its skills. And while it's tempting to leave the two allied ghosts that accompany him alive, since they will take his actions up, most offensive Casters that could take advantage of type advantage have AoE Noble Phantasms. Good single-target offense-oriented Casters are rare; no Servant in the game truly fit that niche when this quest came out, and even now it's pretty much just Sanzang, Illya, and the Caster of Midrash, a rare 5*, a limited 5*, and a story-locked 4* respectively. That said, Robespierre's ghost does have at least one exploitable weakness that the game never tells you about: boosted vulnerability to the otherwise-wonky Instant Death mechanic. Players struggling to chop down his massive health total may find it easier to exploit this and kill him outright.
  • The second to last fight in Tristan's Interlude is quite possibly one of the hardest fights in any Interlude. He starts out with his Reversal Gift from the Camelot chapter, causing him to resist all classes but Saber and deal bonus damage to all classes but Shielder, and gains the Gifts of both Gawain and Lancelot after cracking his Break bar, allowing him to gain two points of Charge each turn instead of one and causing him to take reduced damage from all classes, with further reduction for Noble Phantasm damage. He also retains his full skillset, meaning he can completely fill his Charge bar in one turn with Unblessed Birth (although he thankfully cannot do so and use his Noble Phantasm at the same turn, since it inflicts NP Seal), and spam his Harp of Healing to give himself a free hit of Evade. Also, while his Noble Phantasm is only single-target, it ignores Evasion and deals boosted damage to almost every Servant in the game. The player is also forced to use either Lancelot, Gawain, or Bedivere as support Servants, and while they are Sabers, only Lancelot is really well-suited to the fight, and he doesn't come with his powerful third skill. Any unprepared player without powerful or well-leveled Sabers and/or tank Servants will struggle to pass this barrier, since they cannot rely on their Friends list for support. Thankfully, the next fight has few gimmicks to contend with by comparison.
  • Aŋra Mainiiu's Interlude is a nightmare to play through, though it's likely expected given to unlock it means you need him at Bond Level 10, meaning the developers were confident you had time to grind up for a fight this hard. There are a total of 100 Shadow Servants, with 99 of them being Aŋra Mainiiu himself and one of them being an Archer EMIYA, but only the Shadow EMIYA needs to be beaten to end the fight. The hard part comes in with how the mechanics of the fight work. Each turn, every single Shadow Aŋra on the field gets the buff Annihilation Wish, which translates as a one-turn Attack Up and a three-turn Critical Chance Up, and while they have generally low HP (none that break above 7,500), the buffs stack, meaning one of these guys can actually deal surprisingly high damage thanks to Avengers only having disadvantage to Moon Cancer (one of the rarest Classes in the game), especially if one of them manages to survive to get three of the crit chance buffs on. Once five of them are defeated, Shadow EMIYA comes out, who has three bars of health topping over 300K each, coupled with his one-turn Evade and three-turn Defense Up buff from Eye of Mind (True), which makes it difficult to burst him down even with single-target Lancers. Upon his first Break, he'll cast an unremovable party-wide Curse debuff that hits for 1,500 HP per turn for five turns, and for the second Break will cast a three-turn party-wide Invincibility buff that mercifully stuns him for two of them. And all the while, two Shadow Aŋras will be along for the ride to soak up damage for him if you want to prevent them from getting too many buffs to start one-shotting Servants (and they will if any of them were around when the three-turn Invincibility buff goes off). Dealing with this fight comes with constant micromanagement of knowing when to deal with the Shadow Aŋras and when to attack Shadow EMIYA. About the only saving grace is that none of them can use a proper NP (the Shadow EMIYA can only use the Extra Attack, but even that can eat a good chunk of even a Lancer's health and with a three turn NP gauge he'll be making a lot of use of it given the sheer amount of health he has).
  • Beni-emma's interlude. First, you start off against a group of three Gold enemies, meaning that they can wail on your party for three attacks per turn, and speaking of the party, you're basically forced to use only three story supports and nothing else. Said story supports are Diarmuid Saber, who is a remarkably good servant, Tawara Touta, who is a decent AOE archer, and Fionn Mac Cumhaill... without his Strengthening quests. This means that you're stuck with three servants with one of them being below-average and left at the mercy of the RNG gods who can potentially decimate them. The second fight, on the other hand, while it does still force you to use the same forced supports, this time you (thankfully) are forced to bring in Beni-Emma as well, and face against the raid boss version of Ibaraki-Douji, though in spite of her 6 Guts buffs, defeating her hands ends the battle. Fortunately, if you manage to endure all of these, aside from the Saint Quartz the game rewards you with, you're given a decent amount of QP for it.
  • Dantès' second interlude can easily be described as "Those Two Bosses: The Interlude". For both battles, you're forced to use both an NPC Dantès as well as your own, meaning you're already down to 4 servant slots (3 of your own & 1 support). The first battle is a rematch with Ibuki-Douji from Heian-Kyo and it is every bit as painful as before, and while it is slightly easier due to nerfed stats and Dantès' invulnerability pierce on his 1st skill, you'll still need to prepare a competent backline in case you get wiped out by her NP. The second battle has the return of Cernunnos, infamous for being considered the hardest story boss in the game even with a well-structured team. Since these are interlude fights, you are unable to use any Leyline Stones (if you have any), meaning you'll have to use either Command Seals or precious Saint Quartz to revive your team. Granted, this interlude requires one to complete the Tunguska Sanctuary in order to unlock, but having to fight two infamous story bosses back to back will definitely surprise a few unsuspecting masters.

  • The level 80 free quest for the Saber Wars event had Tesla, an already powerful Servant in an event that encourages you to use Sabers, which Tesla is strong against as an Archer. And his quest was one of the best places to farm one of the event currencies. Hope you had a strong Lancer handy!
  • In the first Valentine event, there were rate-ups for each class that made a Free Quest spawn that particular class. This led to players building teams to counter one class when either a demon, dragon or an enemy Servant of an opposite Class curb stomps them.
  • The 6th Day of the Prison Tower event has the double Ruler combo of Jeanne and Shirou. On one hand, you get a powerful Edmond Dantes as your support, who has the advantage against Rulers as an Avenger. However, before Jeanne and Shirou comes a wave of Caster-class wyverns, followed by a very bulky Rider-class wyvern, who will tear apart the team before they even get to the boss. And Jeanne and Shirou themselves are no slouches, with Shirou in particular having an NP that debuffs before dealing damage, rendering invincibility buffs useless, and necessitating that he goes down as soon as possible. After Amakusa is down, though, Jeanne is very easy to deal with.
  • On the last day of the Prison Tower event, before the final battle with Dantes himself, players must deal with a giant ghost enemy. Its AoE basic attacks hit hard and its Assassin class means it has a high chance to crit, it has tons of HP (500,000+), and has such high magic resistance that most stuns/debuff can't affect it. Also, at the time the quest first dropped, single-target damage-dealing Casters were very rare, without a single 5* caster in the game fitting that niche.
  • Nero Fest's Exhibition battle with First Hassan. Even before the quest released for North America, this fight was already reckoned as difficult, poorly-designed, random, and frustrating on other servers, given First Hassan has a boosted chance to inflict Instant Death with all attacks, making his Noble Phantasm a virtually certain death sentence even through Invulnerability or Evade, and starts the fight with twenty stacks of potent Special Defense that he slowly removes, one at a time, by decapitating his endless streams of teammates at the start of each turn, gaining powerful buffs with each decapitation, and with harder-to-kill teammates granting more-dangerous boosts and being prioritized for his decapitations. But, at the start of each turn, he would plant a single-hit Target Focus on the first servant in line. While this still made the fight very-random (he sometimes leads off by using one of his skills instead of attacking, meaning that the target can be dispelled by one of his teammates instead, and if he decapitates a teammate he'll get to act twice in a round), players could still prevent his rapidly-charging Noble Phantasm from hitting anyone else in the party on turns it was destined to go off (and not only does he not decapitate one of his teammates if he starts the round with a full charge bar, but the hardest-to-kill enemy gives him a beefy attack boost and a free point of bar) and ensure that the NP hit a servant like a friendly First Hassan or BB who was either immune or supremely resistant to Instant Death, respectively. The fight was still long, frustrating, random, and deserved an entry here, but there were multiple strategies for managing it... until it released on the North American server. Now, the First Hassan applies Target Focus to a random teammate at the start of each turn, making the mechanic as a whole pointless and agonizing. The only remaining viable strategies are to stack a team with Irisviel, an Event servant many later players won't have access to whose Noble Phantasm applies team-wide Guts, and some of the toughest, and rarest, defensive supports possible, and/or abusing Order Change and stacking the most powerful possible buffs on a small selection or rare offensive servants (Sanzang, Illya, Hijikata, etc.) to burn him down after his protection weakens... and even these are Luck Based Missions par excellence, requiring lots of extremely-random factors to align perfectly and with countless chances for something to go wrong throughout, given First Hassan's heavy damage potential and chance to inflict Instant Death at random on any lucky hit, Irisviel's well-known issue of having very, very low NP gain, and the, again, low availability of many servants involved. At least he's optional.
  • The final Challenge Quest of the "Summer 2017: Death Jail Summer Escape!" event is the infamous Bull of Heaven, a hard fight on its own. However, before it can be faced, players must first fight Ishtar in her archer form. Like other archers, Ishtar has a three turn Charge bar, but thanks to her high HP, attack, and her skills that buff her damage and boost her Charge while giving her a random chance for both Invincibility and Pierce Invincibility (and is more likely to do so at low HP), players face a serious gatekeeper to even fight the Bull. This not only prevents players from simply bringing Assassin-heavy lineups to capitalize on class advantage, but forces them to fight a high-speed damage race with a dangerous enemy first. Meanwhile, the Bull itself not only has a Rider class to punish powerful Support Casters who might help trivialize it, it deals area damage on critical hits and can boost its own critical rate while draining allied critical stars, can inflict many dangerous debuffs to random card types, and packs Buff removal, both single-target on one of its skills and to the entire group after losing its first Break bar. And while the damage its Charge attack inflicts is not great, it has a chance to inflict NP and Skill Seal after using it, and its second Break bar causes its normal attacks to give it stacking Attack buffs. While a powerful First Hassan can tear up the Bull fairly easily, and many Arts-based non-Caster supports like Jeanne will make the battle easier, starting players without access to such powerful servants can only use their single Support pick on one.
  • Most fights in "The Tale of Setsubun: Oni Pagoda Festival" event are already more difficult than normal, due to the restrictions and gimmicks of the event: no support Servants of any kind may be used, and Servants become "exhausted" after fighting and require 2-4 hours to recover, depending on whether or not the limited number of slots that half remaining recovery time are in use. But even with these in play, a few battles really stand out:
    • Floor 60 has a fight against Osakabehime, Kiyohime, and Tamamo. While at first this seems like a strange team comp, the game flat out cheats and gives Tamamo a special ability she can use every turn that gives a flat NP charge to one of her allies, but the also counts this as not being her attack, so she still gets to act, meaning the enemy gets four actions. The strategy of the boss is to get Osakabehime's NP out thanks to its generous three gauges, buffing the teams defenses and max health, before helping Kiyohime get her NP off. The result is that the player is in a Sadistic Choice: if you try to take down Osakabehime first, Kiyohime will deal high damage to any target that isn't a Foreigner, resulting in her AOE NP getting used to potentially weaken your team, but if you try to take her down first, you'll have to deal with Osakabehime buffing her and her allies defenses to the point where you can't damage them, and all three have defense up skills as well.
    • The 100th floor battle against Shuten-Douji is this for the wrong reasons. She has fairly high hitpoints on her first break bar and, as an Assassin enemy, her natural predators, Caster and Alter Ego class Servants, are stereotypically either poor at dealing single-target damage or rare and limited, meaning even the Event CE might not be enough to crack her. Upon breaking, her next health bar has a paltry ~33,000 hitpoints, but she gains universal 90% special defense for five turns. Her Noble Phantasm is AoE, and while its damage is not overwhelmingly high, it inflicts a massive hodgepodge of debuffs on your servants. Unlike her Berserker-classed Shimousa counterpart, she has the Assassin class's three-tic Charge gauge, meaning she can fire it off very quickly. Alongside her usual skillset (which includes a teamwide Charm and DEF debuff), she can also give herself a free point of charge, meaning she can catch even a well-constructed stall team off-guard with a sudden NP. This all combines to a frustrating and random fight where the biggest factor in victory and defeat is whether or not key members have been randomly stunlocked.
  • "Servant Festival 2018" has a very annoying raid boss in Sessyoin of the Night, which is Kiara as a Moon Cancer. Despite her Moon Cancer class, she still has access to her Nega-Saver passive, which allows her to deal super-effective damage to Rulers. She also puts an unremovable taunt on one servant to burst them down, then switches it to another target when they drop. Once her Raid HP bar is down far enough, she starts the battle with a full NP gauge, and since her Noble Phantasm bypasses defensive buffs, there's no way to survive it without debuffing Kiara. To top it all off, if you try to fight her during the day, she gets an insane damage resistance boost.
  • "Battle in New York" features an Exhibition Quest with a Mysterious Heroine X Alter and 19 extra enemies that grant various buffs or debuffs to whoever killed them, and Mysterious Heroine X Alter will use a skill each turn to kill one enemy. If you were thinking you could just focus on Mysterious Heroine X Alter as the fight will automatically end once she is dead, you are in for a nasty surprise as behind her first 400K HP bar is a whopping 10 mil HP and she gains Break Bar effects to permanently buff her attack, critical damage, critical chance, and charge her NP by two ticks each turn instead of one. Given her Berserker class, getting a critical hit will be an instant death sentence for most Servants, especially since two of the accompanying enemies will buff her critical chance and strength even further for five turns. If you don't manage to get through their 450k HP the turn they arrive, prepare for critical hit hell. But the worst enemy comes in the form of a Spriggan with almost 640k HP that will spell trouble no matter who kills it. If Mysterious Heroine X Alter kills it, she will gain a permanent buff in which her normal attacks remove one random buff from whoever she attacks, meaning she could take an Evade or Guts that could make the difference between survival or death. If you kill the Spriggan the turn it arrives (as she will kill it otherwise), your Servant will be permanently debuffed with a max HP of one, so if the Spriggan was killed by your main damage dealer, hope you have ways to keep them alive. The only saving grace is that breaking her first HP bar will trigger a skill that slowly removes two million HP per turn after 15 turns, but that assumes you can actually survive that long, especially if Mysterious Heroine X Alter got the Spriggan's buff. Oh and while you could obtain extra damage-boosting Craft Essences to potentially have multiple damage dealers with max Limited Broken damage Craft Essences fight her in the original version, the event's rerun only lets you obtain enough damage Craft Essences for one max Limit Broken copy. If you can't use a Support with the damage Craft Essence to go along with your own damage dealer, have fun fighting enemies with huge HP bars using only one Servant with boosted damage.
  • "Lady Reines' Case Files" has some notable hard fights.
    • One notable battle in this event for its difficulty is the Setsubun memorial battle where the player pits against Minamoto-no-Raikou and a bunch of Ibaraki-douji clones. Each turn, Raikou kills a clone which then casts incredibly painful burn debuffs to party members upon death which stacks per clone killed. On Break, Raikou gains buff removal effect upon attacks. And oh, since this is Setsubun, you can't bring friend supports.
    • Robespierre's ghost from d'Eon's Interlude makes a return here with the same high HP bar as before. While the two ghosts that accompanied him are no longer with him, he instead gets assistance from Jeanne Alter... as a Ruler servant. This can lead to a difficult and annoying boss fight.
    • The fight against Mnemosyne is even worse. A Rider with three health bars and in-built damage resistance that not even Gray, who has class advantage, a damage against Undead buff to use on the boss, and an event damage bonus can do much damage on her with a Brave Chain (and note that this is the last fight before the story ends so she's stuck with NP Level 1). It pretty much demands getting Gray to fire off her NP as quickly as possible if only to drop and stack the Buster and Quick Resistance Down debuffs on her. She also summons a Shadow Da Vinci who can kick your Assassins' balls (most likely Gray's) real quick, which doesn't even cost an action in addition to being able to do it infinitely; the worst thing on top of using both NP and skill seal debuffs is that she fires off her extra attack after casting party-wide buff removal, meaning all invincibility/evade buffs used beforehand become worthless. The saving grace of this fight is that all of its attacks are AOE which helps fill up the party's NP gauges, if they can survive long enough, that is.
  • The Elisabeth Báthory bounty quest in "Saber Wars II" is regarded to be extremely hard to the point where some players believe it was similar to a challenge quest. To start things, Saber Elisabeth constantly likes to spam her Heroine's Great Principle skill which grants her invincibility and a NP charge, and after breaking her first break bar, she gives all three Elisabeth's invincibility. This pretty much forces players to use a CE/Servant that can Ignore invincibility. They all have different amounts of HP, meaning you have to account for the fact that they all are most likely not going to take the same amount of damage before their Break Bar is removed and come out similar enough to make one Servant a catch all (Siegfried for example). To make matters worst, once you defeat one Elisabeth servant, they shall cast a Delayed Buff on another Elisabeth servant where if you failed to defeat that Elisabeth servant in five turns, the Elisabeth servant you defeated shall be revived. To make matters, since the three Elisabeths on the field are in different classes, the only servants with class advantage against them is the Berserker class, but if you bring one of the Berserkers there, the Elisabeth's might be able to wipe them out in a critical hit. Overall, this is a difficult and annoying bounty quest and it’s quite telling when it’s the only Special Bounty to not give out a Goddess Scripture nor a Command Code.
  • Imaginary Scramble:
    • The Main Interlude's version of the first proper fight against Enemy Ship Alpha is a major pain. For one, you're forced to use only NPC servants. While the event version did at least give them event damage bonuses to compensate, the Main Interlude version doesn't give any due to the way Main Interludes are supposed to work, and the Enemy Ship Alpha's HP was not adjusted to compensate for that. This means, even with two Foreigners on your team, the Enemy Ship Alpha's crit and ATK buff per-turn may overwhelm your team while they deal in practice the damage of an wet paper towel. Oh, and Van Gogh's most useful trait (her NP) is sealed, meaning you can't use her to cheese the fight. It says something when the other Enemy Ship fights don't force you to use an set of NPC servants and allow you to freely use other servants, making them easier by comparison.
    • The battles with the Shadows of Van Gogh in before the final battle sans Water Nymph’s Self-Punishment are difficult in their own right with all except the first forcing you to use an NPC in place of Friend Support, but the hardest would have to be Water Nymph’s Regret. The gimmick of the fight is that for each turn that passes, the boss will get gain a Guts buff that will last until removed in any way and grant her an Attack Boost with each Guts that was removed in combat. That is to say, these will continuously stack as the fight goes on and it absolutely will unless you have an Alter Ego or Foreigner, all of which are hard to come by and strong supports of your own, on top of her having basically over 900,000 HP. The battle will most likely require you to have someone who can remove buffs from their foes but Water Nymph’s Regret, upon Break, will place three unremovable Guts on herself, (which thankfully take priority over her regular Guts when her health is dropped to 0 and all the Guts only restore 3000 HP upon use) not only prolonging the fight further, but ensuring she will get those attack boosts and force you to have to inflict Buff Removal at a specific time if you don’t want to deal with the extra Guts she’s built up while gaining further attack buffs.
  • The final boss of the Tunguska Sanctuary Beast IV:L Tamamo Vitch Koyanskaya takes everything about the Beast of Taming in Olympus and ramps it up to eleven. She still retains her Nega-Weapon passive (Deals effective damage to Hominidae servants, Less damage to Casters and is weak to Wild Beast servants), but she gains a new Nega-Self passive, which lowers the buff success rate of everyone except for herself by 40%, meaning that all your servants have a significant chance to whiff their offensive & defensive buffs (Unless you bring along the NPC Tai Gong Wang). Upon breaking her 1st bar, she increases the cooldown of all your servants' skills by 3 turns & applies a 1 turn skill seal while granting herself a hefty 50% defense buff. Breaking her 2nd bar lowers your team's defense and grants her a chance to ignore invincibility buffs when she attacks. Her 3rd break lowers your team's defense again and her NP gauge will charge twice as fast. But the real threat in this battle is her entourage of retainters, each with their own unique effects guaranteed to make things as tough as possible for you. Defeating these retainers or Breaking the boss' HP bar will grant her an unremovable 50k bonus to max HP per retainer, which means her final HP bar can reach ludicrous levels. While all annoying in their own right, there are two notable retainers to watch out for; The first one, Retainer V, appears after breaking the boss' 1st HP bar, and increases the NP damage resistance of all enemies (except themselves) as well as granting them a 2-hit invincibility every turn. Meanwhile, her last HP bar has HWB-M8 which is the most dangerous of them all, as attacking anyone else except them inflicts instant death on your entire front line, forcing you to single them out while the rest of the enemies are free to trash your team, and since said enemy is a Ruler class, it is very likely your team will fall by the time you finish dealing with it. The Main Interlude version of this battle is even worse, since there's no damage bonuses available and while the enemies are slightly weakened, they still reach nearly 100k HP. Your only saving graces are the occasional outside support from Ibuki-Douji and that your party has an unremovable and replenishing Guts (with a 5 turn cooldown).
  • The final boss of the Nanmei Yumihari Hakkenden event, Kyokutei Bakin is a surprising jump in difficulty compared to the previous battles. For one, they start with their NP at full gauge, and since they have an 8-turn Debuff Immunity, this means that NP Seals, Stunning and NP ATK down Debuffs are useless, forcing players to include Servants that can provide Invincibility to prevent them from instantly one-shotting the entire party. While draining their NP gauge might seem like a good idea, on their first break bar, they gain a buff that allows them to gain an NP tick when attacking, effectively counteracting the NP drain in the process. Finally, on their last break bar, not only they gain an 8-turn 50% ATK and DEF up buff, but also remove all of your buffs, meaning that unless you bring a servant with Buff Removal Resistance, even if your party does survive the NP, they'll being doing virtually Scratch Damage while the boss can 2HKO them in return. Not being helped is that before then 2 waves are fought before them, meaning unless you bring 2 servants to sacrifice themselves to take down the waves, you will be wasting turns trying to get to the boss.
  • The true final boss of GUDAGUDA Neo Yamataikoku event, Neo Sen no Rikyu, is basically the above boss, except she trades in the above's debuff immunity for a massive defense against Buster and Arts cards, forcing players to use Quick servants to deal meaningful damage. Also, should she suffer a critical hit, she will debuff her attacker with an stargen down and star absorption debuff, ensuring not only they won't generate a lot of crit stars, but also prevent their command cards from gaining well-needed crit stars as well. Upon her first break bar, she'll gain an Invincibility buff that can only be removed by waiting 3 turns or hitting her 3 times with the buff, and the only way to bypass it is to get a servant that has Invulnerability Pierce, and the only two servants that do have it are Paris and Altera the San(ta), with the former's low damage output may be not enough, and the latter's skill which grants Invulnerability Pierce has a chance to stun herself after using it. Most infamously, much like the above boss, she will remove all your buffs and her debuffs, and no Quick Servant has Buff Removal Resistance, meaning that you have to mess up your team composition to keep your buffs unless you want your damage dealer to deal the equivalent amount of a wet paper towel while she obliterates your team in return. Due to this, she quickly earned the title as one of the hardest non-Challenge Quest bosses, only surpassed by Demeter and Cernunnos.
  • Reaching the final Exhibition Quest in the Chaldea Faerie Knight Cup event has Morgan, already That One Boss in Lostbelt 6, come back for another round and is even meaner than last time. It's a sort-of gimmick fight where all the staffers for the event show up to give Morgan some very scary buffs upon arrival (so no, you cannot stop them), and she starts with 99 damage reduction stacks. The good news is that if you defeat one of the summoned staffers before they flee, they'll also give their buff (or an equivalent buff) to your frontline and remove some of Morgan's DR stacks. The bad news is that they show up two at a time and you only get one turn to defeat them, requiring an in-depth knowledge of the game's targeting mechanics if you don't want to miss buffs or let Morgan keep too many DR stacks. Morgan's NP is thankfully sealed until all the staffers are gone, but once the fight starts proper, Lordless Camelot comes out immediately, and among her buffs are an increased NP gain that basically enables her to loop — and remember, this is a Berserker AoE NP coming from a Servant who gets extra damage against anyone with the very common Man trait. And just like LB6, she brings two horrifying Break Bar mechanics: the first slaps Evil Curse, NP Gain Down and NP Charge Down on your entire party (even the backline!), and the second gives her some more flat DR stacks and steals half of your frontline's NP gauge to charge her own, and just two active Servants is enough to give her a full meter. The only mercy Morgan gives you is that she lacks her buff/debuff manipulation mechanic from LB6, incentivizing you to keep your staffer-buffed frontliners alive at all costs, because anyone who missed the buffs will get popped like a piñata.