This page is for venting on bosses that gave large numbers of people grief. They do not need advice on how to beat them. They already know how to find GameFAQs.
The page isn't for you to talk about how great you are at the game either. Save that for your blog.
NOTE:Final Boss and Wake-Up Call Boss cannot be That One Boss without being overly hard by their standards. Please do not add them as examples. Bonus Boss is completely banned. They are supposed to be overpowerful and have no real standards.
In stand-alone gzdoom mod of Doom called scoredoom, only in Ultra-Violence mode when playing with the Add-on pack will the player see different bosses, including variants of the Cyberdemon and Spider Mastermind, all of which are very much this trope (save for the dark avatar).
Maledict in Doom 3: Resurrection of Evil. You start the battle by landing on a flowing platform in the bottomless space of Hell. You have the the Artifact but it's only in limited use. Maledict demands you to hand over it, but the marine simply points the gun at it. The battle start with Maledict randomly throwing fires and summoning the local cannon fodders at you, which you just simply kill and then use the Artifact to slow time and just fire whatever kind of guns right at Maledict's slow-flying ass. After a while, Maledict decides that its gonna stop bull-shiting around and then just throws meteors at you, and all you can do is to avoid them and not trying to fall over the platform, as well shoot the beast up. And you can't get any healthpacks during the battle.
Cthon in Quake is a normally invincible Puzzle Boss that is defeated by three switches, when all previous enemies had been killed by brute force. You must run across narrow catwalks over lava to reach these switches, and Cthon tosses instant-kill lava balls at you that on higher difficulties are fired at where Cthon thinks you will be when they hit, and Cthon's guesses are uncannily accurate.
Tomb Raider: The Angel of Darkness: Boaz, mainly due to the buggy target lock-on controls. What makes Spider Boaz really difficult is that up to this point, every boss could be killed by just pumping enough ammo into them while avoiding their specials. With Boaz, you have to target her sides when she does a specific move, preferably without getting hit by said move. The game didn't exactly teach you this, and doesn't tell you before or during the boss fight.
In Halo 2, the Heretic Leader, particularly on the Harder Than Hard Legendary diffuiculty. Equipped with a jetpack, near-instant-death dual Plasma Rifles, and multiple holographic doppelgangers, all of which have jetpacks themselves, and can somehow deal the same amount of damage that the real one can. Definitely worse than the Prophet of Regret.
The Death Knight in Spear of Destiny is by far the most difficult boss in the game, packing more than twice the firepower of any other boss. To make matters worse, he's in a room with at least twenty mutants, guards, and SS in it, and no health or ammo. To replenish health and ammo, you have to run through this room and out a doorway, and into a ring-shaped corridor running around the level... which is full of officers. There is a better than even chance that you will die within seconds of entering the boss arena, before you can even get out into the outer ring and lure the boss into following you.
In TimeSplitters 3: Future Perfect, the Creature (Crow's monster) in the episode "Something to Crow About". The worst part is the collision detection for his body is wonky: So you step out from behind one of the ballroom pillars and open fire, hoping to take out one of his weapons. Even though it was a direct hit, the weapon didn't notice and is now unleashing hell in your direction, knocking you back into the proximity mines he laid, which blow you away from your cover so that you're now taking fire from multiple weapons. Rinse, repeat. And if you run out of ammo for both your minigun and your rocket launcher, you might as well put down your controller and wait for the boss to kill you, as nothing else you have is going to make a dent. Mercifully, there is a glitch that causes him to start the fight with zero health if the player skips the boss intro cutscene at the right moment (when the boss begins to raise his "arms").
NME (or Nasty Metallic Enforcer) from Rise of the Triad is undoubtedly the hardest boss in the game (Yes, even harder than the final boss). Not only does the damn thing move very fast, but it also hurls mines and heat-seeking missiles at you. The developers outright state in the full-colour printed manual: "We even have trouble beating this guy." Extreme Rise of the Triad has levels where you fight NME in a small space. Good luck beating it in those levels.
Killzone 2, the first ATAC gunship fight (the second time it doesn't even really count as a boss fight). The ATAC is faster than The Flash on methamphetamines, has a fairly painful machine gun turret, and has rockets of +5. And your only source of cover in this fight is a single stone column in the middle of the area. Which offers you no protection at all from the splash damage of the ATAC's rockets, and is so thin that taking cover from the machine gun entails getting so close to the column that you can't see where the boss is. Basically, success in this battle comes down to getting in enough hits before the thing decides to spam its rockets and instakill you.
The Big Daddies of BioShock can frequently provide quite a difficult fight.
Bouncers may seem to be lumbering idiots, but once they turn against you, things go very nasty very quickly. Don't underestimate them because they have no long range attacks. They like to root you into place with a paralyzing shockwave and then ram into you at near superhuman speeds, which is pretty much a projectile attack. And unless you're playing on Easy, each attack is probably going to require you to use one of your precious First-Aid Kits.
Rosies are even worse, because they have a projectile attack. Their huge rivet guns take large chunks out of your health, enough that healing will be necessary until you've bought a few health upgrades. Plus, they like to fire proximity mines while you're hiding, just to turn the tide in their favor and making it more difficult to move around freely. The mines have a large radius, can't be picked up by TK once they're set, and even stay after the fight. What fun!
The Minerva's Den DLC for 2 has the Lancer Big Daddies, which chase you for the entire fight constantly burning holes in you with their powerful Ion Laser. They are by far the hardest fight in the DLC, especially as you can deal with the Big Sister by Hypnotizing a Lancer and letting it take a chunk of her health.
The Siren is the only real boss fight in BioShock Infinite, and it more than compensates. It doesn't have a lot of health but it's immune to most of your Vigors, and constantly flits around the battlefield making keeping track of it - let alone actually hitting it - a minor miracle. It only stays still for a few seconds so it can resurrect corpses as enemies; they're not weak enough to just ignore, but by the time you've killed them The Siren will have gone to another group of corpses and resurrected them (and by the time you've killed them The Siren will have gone to the first group and resurrected them, ad infinitum). Using Devil's Kiss or Shock Jockey to disintegrate the corpses helps, but sometimes The Siren will just go ahead and resurrect that pile of ashes. The best part? You have to fight this monster three separate times.
In Clive Barker's Undying, there's the penultimate boss, Bethany. She's widely considered to be much, much harder than the actual final boss, and has marked the point where many players have quit the game in frustration. Why? Well, for the one thing, she flies, meaning that in order to get up close to her and perform the one melee attack that can kill her, you have to manage the awkward flying controls and fighting controls at the same time. But that's not the worst thing. Oh no. She also summons enemies to fight for her while she flies around overhead. Enemies that can kill you in more or less one hit each. Enemies that respawn endlessly until you kill Bethany herself. And the plural term "enemies" doesn't just refer to the fact she'll summon them repeatedly: it's that she summons multiple one-hit kill enemies at a time, who'll pile onto you while you're trying to get to her to put a mercifully quick end to the battle. And since she's positioned right before the final boss, if you lose to it and quit the game, you will have to deal with her again.
The Preacher fight from S.T.A.L.K.E.R.: Call of Pripyat. After launching an elaborately planned ambush on a Monolith patrol, with sniper support and a squad of specially trained, heavily armored Spetsnaz, he announces his presence by instantly killing said sniper, and the real Monolith patrol shows up. Suddenly, you and your six man team are pinned down in the middle of an open courtyard, outnumbered nearly 3:1, with sniper fire raining down on you. Even worse, if you take too long the Preacher will start using the Cool, but Inefficient Gauss Rifle. Even worse, even the best armor in the game will be instantly ruined by a single shot, and if you aren't wearing that armor, you die instantly. If he can't find you, he'll simply fire on your allies, and is capable of wiping out the entire squad singlehandedly in under fifteen seconds.
Somehow, to make things even worse, in order to get a good ending, three specific Spetsnaz out of the six absolutely cannot be allowed to die. This doesn't sound like too huge a problem considering what you've been doing all game, but two out of the three mission-critical guys are going to be out in the open when the Monolith guys show up. Even if you take out the Preacher immediately, you have to hope the ridiculously heavily armed mooks don't take down a single one of your important buddies. And if you take too long, one of the mooks will pick up his gun and use it on you.
The game has another situation later in the game, during the final mission, where you must escort a team of story-critical characters to the evacuation point. The catch? If you lost any men in the mission above, the survivors are weaker as a result. Half of them are outfitted with pitiful starting game gear. This Escort Mission from hell gets even worse when another Preacher (using a powerful semiautomatic shotgun) appears at the very end (along with 40 more Monolith mooks!).
Packrat Palooka from Oddworld: Stranger's Wrath, who is essentially an upgraded version of a rocket-firing outlaw. Most of your sucess depends on you managing to knock him off of his platform with Boombats or ThudSlugs, and then giving him a good arse-kicking before he scrambles back up. All whilst he continually fires homing rockets at you, which can kill you in just a few shots. And that's not even getting into trying to capture him alive.
In Borderlands, Mothrakk is a very challenging fight for its level. For one thing, it flies very high up in the air, meaning grenades and special abilities are very hard to use to deal it damage. While it is an enormous target (dwarfing several buildings nearby), its flight speed and distance make it hard to land hits with rockets and other slow-moving projectiles. It also has no critical hit point and a huge chunk of health, so huge that with an average gun by that point it could take five minutes or more of sustained fire to kill Mothrakk, and that's without missing. Its attack is to fire off these fast, unpredictable barrages of fireballs that tear through shields and deal massive damage as well as significant knockback. Even with a shield that resists fire, three barrages in a row can kill even tanked-up classes. And if you try using a vehicle, you'll find it's a pain in the ass to lock on, that the rocket launcher is completely unable to hit it because it's too slow, and that the machine gun does piddling damage. You also lose a lot of mobility in the valley area you're in when commandeering a bulky car. There really is no surefire way to kill Mothrakk other than finding a decently powerful gun with the game's enormous weapon randomization and then praying you won't run out of ammo.
Borderlands 2 offers the Badass Pyre Thresher, a type of worm with a lot of tentacles. Whenever it emerges a large area is engulfed in fire, which will almost always break your shield and cause significant damage to your health. After this, it will immediately lash out at you with its tentacles, killing you. It will only emerge in places where you are in range of its fireball, and will retreat underground if you cause significant damage, only to emerge again and finish you off. Its sizable health bar is fully replenished on every respawn. Unless your gear has the raw DPS to kill it in two seconds, you are not fighting it. You will instead be sprinting through its territory, hoping its explosion will knock you out of its tracking range, screaming profanities and prayers.
In killer7, there are two bosses that are much harder than the other bosses at least on the first attempt. The boss of Cloudman, Andrei Ulmeyda as a Smile, forces you to navigate through a maze and sneak behind or to the side of his afro to shoot it once. Ulmeyda, however, has a special AI due to the type of arena he's fought in and is the only enemy in the game to be able to hear your bullets if you miss and come after you. Also, his main attack is extremely long distance, with his field of vision for the attack increasing with the difficulty level, and if he catches you, it's a one hit kill. He's a nightmare in another way, as well, to make the fight even harder.
The next major boss, AYAME Blackburn in Encounter, Part 1, takes forever. She runs around a giant parking lot faster than even Con without the speed boost and even rivaling him with it on higher difficulties and can only be shot under one of the streetlights. She has a fair amount of health, as well, and attacks with a machine gun that fires bullets that seem to be made out of energy.
The last two minibosses (though the first can arguably be considered a normal boss due to a regular boss not being present in its level) are worth noting. The Timer Smile forces you to hit certain parts on its body before it reaches you, which is easy... unless you didn't save a very easy to hit target, like the one on its head, for last, as once there's only one left, it Turns Red and runs extremely quickly. Then, the Galactic Tomahawk Smile fires an unrelenting barrage of missiles that start becoming more and more frequent as it loses health and, once one hits, they all start coming and you have to start mashing the pause button to quickly heal.
Team Fortress 2: Each Halloween event since the second has spawned a raid-boss-like monster whose health increases based on the number of players on the server, has bullshit attacks that hurt like hell and pretty much can't be dodged, and can only be defeated if everybody on both sides set aside their differences and focus fire on it.
The Headless Horseless Horsemann, the very first in a line of bosses, has an axe strike that is pretty much always a One-Hit Kill. There are only two things that can survive it: Dead Ringer Spies or a Wrangler-shielded level 3 sentry, and he'll attack again right after that. He can also scare you, slowing those affected and preventing them from attacking or jumping. And this is all accompanied by the fact that once he kills somebody, a random person is marked as his target. They can only transfer the mark by hitting somebody with their melee weapon. If they're slow, they're pretty much screwed. Also, he randomly appears out of the control point currently open. Imagine breaking through the defenses with barely any time left, miraculously holding off the enemies... and then have this guy rise out of the control point, completely throwing off any and all concentration. If you want a special item, you have to deal a hit with a melee weapon during his death animation.
Then there's MONOCULOUS! Unlike the HHH, it is highly mobile and teleports all over the place. It fires slow-moving rockets that deal incredible damage and knockback; hit MONOCULOUS! with a Critical Hit, and these rockets fire and travel fast. You can stun it by attacking it after going through a Death Course that can be accessed whenever it teleports, but said course slowly saps your health and has insta-kill water, not to mention cruel players. And the worst part? At least HHH was there for an unlimited time; MONOCULOUS! only appears for 90 seconds at a time. Even after killing him, you still have to travel through a small area to find a special item, which is also rife with cruel players (and will explode after a while, killing you instantly; of course, you have to stay there for a few minutes to even have that happen).
The worse of these? Merasmus. He has so much health that the previous two bosses are made of glass in comparison. He also teleports a lot, will knock you a far ways away (if not outright kill you) with his staff, and loves spamming the fire launch attack. This attack knocks you in the air, lights you on fire, and he says a line that would make Robotnik proudevery time he used it (at least until more voice lines were added). He also will throw bombs, which deal a big chunk of damage and have no damage indicator (other than obviously losing damage or pain voice lines), so you don't know how much you're hurt until it's too late. Then he will teleport to the control point and rain the aformentioned bombs everywhere for about 8 seconds. So you've got him down to half-health. He then starts taking a strategy from Prophunt and spawns various props all over the map, one of which he's hiding in. These props take a surprisingly high amount of damage before being destroyed, and can hide in the most devious spots. And if you're not lucky... he'll do it again right before he's about to die. Randomly, your head might get turned into a bomb, which requires you to scramble at high speed without being able to jump or attack over to Merasmus so you can run into him and stun him; if you spawn in the prop-hunt stage, you're screwed. This is all compounded by the fact that he is only present for 90 seconds. Combine that with his ludicrous health, and absolute team cooperation is not just neccessary, but absolutely vital to even get him down to half health. Even when nerfed some (he takes more damage from some classes), he's easily the most difficult Halloween Event boss so far. Bring the Huntsman; you'll need it.
The level 16 boss from Descent 2, most commonly referred to as the "ice boss". He doesn't have as many hitpoints as the final boss and his weapons don't do as much damage, but he is armed with homing flash missiles that are nearly impossible to evade and will completely blind the player, making them a sitting duck for the boss and his minions to finish off. The typical strategy for defeating him involves a lot of tedious camping with guided missiles.
The Fan RemakeBlack Mesa has that damn helicopter in the cliff portion of "Surface Tension". It requires a ludicrous amount of rockets to destroy, will kill you in seconds if you're out in the open when it fires, and has a rocket spam attack that can hit you behind cover if you aren't carefully positioned.
Flaahgra, the second main boss of Metroid Prime, is an enormous mutated plant-beast with horrifically deadly attacks, including swinging plant-scythes, shooting various kinds of energy, and summoning a row of plants that you can't jump over no matter how hard you try. To make it worse, he's surrounded by a pool of toxic water that will cause massive damage if you enter it. And the mechanic to beat him is a nightmarish aversion to Boss Arena Idiocy: you have to shoot at the base of giant mirrors that are reflecting sunlight onto him, which renders him vulnerable to attack. The problem is that Flaahgra knows this weakness, and, if you're too slow, will flip the mirrors back into place, making you start all over. By the battle's end, you have to do this with four mirrors. If you haven't been seeking out Missile Expansions and Energy Tanks, it's not going to be pretty.
The Boost Guardian in Metroid Prime 2: Echoes is the best example of this trope in the trilogy. The entire battle takes place in a small room that is constantly draining your health, and half his attacks are nigh-impossible to avoid. There are a few strategies that help a little bit, but the only real way to beat him is to keep at it til it's done. Was massively toned down in the Wii re-release.