Malboros. Their Signature Move Bad Breath inflicts various status ailments, usually all of them, and they're very good at keeping you from doing anything at all until they kill you, if you're unprepared.
In Final Fantasy II four Great Malboros spawn on the first floor of the Second-To-Last Dungeon. They can kill the entire party before the player can even cast Esuna/Recover. And Esuna is not 100% effective, anyway.
The Count Malboros in the Lair of the Father in Final Fantasy IV: The After Years are made worse by the fact that you only have two party members, one of whom has a pitiful 190 MP. He's also your healer, as your other party member can't use white magic. You also can't escape from a battle with one. You will cry.
In Final Fantasy VIII, Malboros almost always act first unless someone in your party has Initiative equipped, and they always open with Bad Breath. Unless and until you can use the Junction system to make your characters immune to, at a bare minimum, Berserk and Confuse, running to a Malboro is entirely likely to result in the player watching helplessly as the party slaughters one another or dies a slow death to time-delayed petrification, poison, and the Malboro's acid spit.
In Final Fantasy X you only have to fight Great Malboros one at a time. Great! But the Great Malboros that infest the Omega Ruinsalways get to act first, and have an 80% chance of casting Bad Breath in their first turn. The only reliable way to survive them is to keep someone in your active party equipped with a First Strike weapon (allowing your party to act once before the G.M. makes its move), and then use your extra turn to either summon an Aeon as a meatshield, or Flee. Powerful characters and weapons only make things worse, as Bad Breath inflicts many status effects including blind, poison, and confusion. Because poison is actually useful Final Fantasy X, they'll be losing 1/4 of their maximum health as they swat at one another fruitlessly or swing too hard and kill their fellow party member. If you're ambushed with Bad Breath, and you don't have any resistant armor or a weapon with First Strike equipped, it's entirely possible to die without ever having touched the controller.
Tonberries. They're slow, sure, and you can run away, but if they get close, a party member is pretty much going to die in one shot. The Genre Savvy player knows to always have a character that almost never gets the final blow on an enemy—Tonberry's Signature Move, Grudge or Everyone's Grudge, does damage based on the number of kills that character has had. Have a healer that almost never gets the final blow, and Tonberry can't do a thing to them, leaving them free to set up/revive the others. Unfortunately, in some installments, they also have an ability called "Chef's Knife", which will kill you deader than dead if Everyone's Grudge doesn't work.
In Final Fantasy VI, Tonberries (called Pugs in the translation) were even worse. Grudge didn't exist yet, so Tonberry's signature move was instead an attack called Step Mine, which did damage based on the number of steps you have taken. While this usually isn't too bad, it has the side effect of removing Vanish status, allowing Tonberry to hit you with his ridiculously powerful Cleaver attacks. Try to attack with magic, and he counters with a Pearl spell. Try to imp him, and he gains automatic critical hits. And to win the best armor in the game, you have to fight three at once.
In Final Fantasy X, the one thing you could use to "hide" from many other encounters in-game was your summons. You try summoning an aeon in front of a Tonberry, the little guy makes a clay effigy of it and stabs it with the knife, insta-killing it.
Mentioning Tonberries around veteran FF players will generally provoke the same reaction as Murlocs for a ''WorldOfWarcraft'' player. Despite this, several FF Fans were disappointed with the lack of Tonberries in Final Fantasy XII, and the weakening of Tonberries in Final Fantasy X-2.
Those who complained about Tonberries being weak in Final Fantasy X-2 must not have made it to the lower reaches of the Via Infinito, where Mega Tonberries start showing up. Just to start, they're not only visible on the regular dungeon crawling screens, they literally fill the entire screen. They can be avoided, but only with extreme wall hugging/jumping maneuvers and it's often not readily apparent from the camera angles. When you do engage them? These suckers are level 90 at minimum (99 when Oversouled), have ridiculously high HP, Strength, and Magic stats, and unlike every other Tonberry known to Final Fantasy, are extremely fast. The Oversouled Mega Tonberries have a version of Karma that does not only an assload of damage, but also Stone status. Immune to Stone? You instead get unblockable Confusion. Confusion in X-2 is ESPECIALLY dangerous; not only can your party members whack each other to death, they'll randomly use skills and items on random enemies and allies. That Megalixir you're saving for the next boss? Prepare to kiss it goodbye. The first time you run into one, it will very likely be by accident, and you will die. Fortunately, there's actually a relatively quick and easy way to kill them, but it involves careful planning around a specific accessory/Dressphere/Garment Grid combo. Even then, it's risky and not always foolproof, as it requires one of your characters to be at perpetually low HP to execute a Desperation Attack. And then the trick got nerfed in the PAL and International versions.
Bonus points for sadism: The oversouled Mega Tonberry is the only source for the ultra-powerful Blue Bullet ability "Cry in the Night". For the player that wants everything...
And the aforementioned Tonberry encounter is just a single one. A later mark puts you up against three at once. Have fun.
Final Fantasy XIII-2 brings them back, and modifies the Grudge mechanics a bit. They start out stabbing everyone like before (and causing poison with every stab), but then you'll see 'Revenge' flash above its head, and it'll start using a more powerful attack called Sharpened Knife. Then, it upgrades to Grudge Knife with the message 'None are safe...', and finally Rancor Knife (always hits for 9,999 damage) with 'Vengeance is nigh!'. Survive all that without killing it, and it'll use Ultimate Grudge, which deals 66,666 damage to everyone on the field. Don Tonberries, meanwhile, can go into Super Mode if you leave them alive long enough, in which they cause hefty Wound damage and get heightened resistances. They're slightly less dangerous than the regular Tonberries, but still a serious threat.
Ghasts and Ghouls have a paralyzing attack, but the game doesn't give you any items to de-paralyze party members, and if you're ambushed by a whole crew of them (which usually only happens in or around the Marsh or Earth Caves), you can end up with your entire party paralyzed and unable to do anything, including the White Mage whose Harm spells are your general standby for dealing with these guys. If that happens, you're left with little choice but to pray that at least one of your party gets out of paralysis long enough to get the hell out before they kill the whole party.
Sorcerers (renamed Mindflayers in the remakes). The entire Ice Cave is the hardest part of the game because of these bastards. At effectively the halfway point of the game, these Dungeons & Dragons monsters do a 4-hit attack which does only about 1 damage per hit. It also has the chance for causing instant death— and your ProRings, which are supposed to protect you against stuff like this, don't do squat against it (because they're eating your brains). If that isn't bad enough, the Sorcerers' other attack is a paralyzing attack that hits everyone in the party. That means they can paralyze your party then pick you apart. And the best part? They come in groups of 4. You are supposed to flee from them— unless you get ambushed by them, in which case you're lucky to even get to your first turn. They also happen to pop up in two other locations: the Lufenian Sky Castle, right on the same small bridge where the most dangerous enemy in the game—WarMECH— can be encountered; and the next-to-last floor between Tiamat and Chaos in the Temple of Fiends, 2000 years ago, where Sorcerers come in groups of nine. Even a max-level party armed with Excalibur, the Masamune, NUKE and FADE is taking its life into its hands by going into that floor.
The Sorcerers' lesser kin, the Wizards/Piscodaemons, bear the distinction of being one of the very few enemies that you can't flee from. Ever. When you first encounter them (in the infamous Marsh Cave, no less) Wizards can shred almost any class other than Fighters, and by the time you reach the Ice Cave you randomly encounter Wizards in groups of at least five or six. If you're unfortunate enough to lose party members to the Sorcerors, better hope the Random Number God doesn't toss in the Wizards to finish the job.
In the same dungeon, half the battles that don't involve Sorcerers tend to involve Mages, who cast instant-death magic that generally hits 100% of the time. They travel in large groups, and frequently ambush you. In the remake, their instant death spell was replaced by the very damn powerful at the time damage magic Firaga, which hit the entire party, and could kill a fully healed party without too much trouble if they managed to ambush.
Ghosts from the Sea Shrine. They can appear in groups up to five and have attack power nearly equal to that of the final boss, and like their lesser cousins can also paralyze you when they hit. It's all too possible for most or all of your party to end up stunned while they hammer you into a fine paste, especially if you're surprise attacked.
Special mention goes to the Cockatrices and Pyrolisks that start showing up in the Earth Cave and Gulg Volcano respectively and show up in many dungeons after that. Pyrolisks have an instant death attack while Cockatrices have a petrification attack that is just as deadly. They tend to appear in large groups and if they don't ambush the party, they still have a good chance of attacking before your mage can get off a group-hitting spell to wipe them out before they do the same to you...
The Death Riders in The Very Definitely Final Dungeon also qualify, as they almost always hit multiple times with each attack, and the damage they do with each hit is directly proportional to your total amount of HP.
Anything that has the weakness "Dark Blade", at least in the DS remake. They are tough as nails, do quite a bit of damage, and you can't hit them normally if there's a gap in the row without them "Divide"ing and making a new one, not to mention they have another move called "Multiply". Guess what that one does? Thankfully, they don't Divide if hit with magic.
In the DS version, the player will encounter frequent spawns of Flame Hounds in The Tower of Babil. They are highly resistant to status effects and have tremendously powerful physical attacks. The pinnacle of their cheapness is a nasty AOE flame attack that can hit your lower HP characters for half their health. They can spawn one at a time, where they are manageable. But they can also spawn two at a time, along with another creature that can turn any of your party members to stone, which makes for a really harrowing battle. Worst of it is, they can spawn in groups of three, an encounter that is nigh impossible to even run away from, even for a party that just easily kicked the crap out of Golbez. It usually starts with each one of them casting their AOE attack in rapid succession, resulting in multiple dead party members, with the survivor or survivors being devastated. If it's a surprise attack, you may as well reload the game. All this while being sufficiently leveled to tromp the ever living hell out of anything else the dungeon can throw at you.
This also applies to the initial versions, to a lesser extent.
Same game, Mages in the underground of the Moon. Especially packs of three that surprise you. Blizzaga, Blizzaga, Blizzaga. DS, meet wall.
The Armored Fiends, also in The Very Definitely Final Dungeon. They have Flamethrower (an absurdly powerful fire attack on the whole team), Beam (an even more absurdly powerful, non-elemental attack on one person), and most annoyingly, Self-Destruct, which will not only knock out whoever gets hit with it 90% of the time, but will also result in you receiving NO EXPERIENCE OR GIL for all your hard work. Annoyingly, their turn always seems to come up right after you get them under 9999 HP, at which point they will almost always Self-Destruct. The worst part? They're one of the carriers of the added Tail items that you can trade for nice equipment, but have a 0.4% drop rate. That means hours upon hours of fighting these bastards if you like to aim for 100% completion. They're fortunately vulnerable to Slow and Sap. And if you don't Thundaga them, they self-destruct less often.
The Blue and Red Dragons of the last dungeon have devastating attacks that hit all party members, and worse yet, are not weak against the elements that fire- and ice-element monsters would be weak against. The blue ones aren't so bad— they can be Stopped, and Stop will always connect, leaving you to heal from Icestorm (a nasty hit-all ability) and beat on them at your leisure. The Red Dragons, however— they have two attacks: a standard physical attack that will hit the tank for around 1000 points of damage, and Thermal Rays, which hits the entire party for around 2000 points of damage a piece. To put this in perspective: the tank will have (unless you augmented him) around 4500 HP. The mages will have around 2500 HP. The Red Dragon is immune to Stop, but can be Slowed, and also Berserked to make him use only his physical attack (which will now hit for 2000 points of damage). The trouble is, though, that the person with access to Berserk and Slow is also the only person with access to the healing spells, so in practice, fights go like this: The Red Dragon casts Thermal Rays, everyone scrambles to heal and rezz, the healer finally gets an open turn, the player goes to select Slow or Berserk from the menu—the Red Dragon uses Thermal Rays AGAIN, the party drops everything to heal, rinse and repeat until death.
The last dungeon also has the Wicked Mask/Deathmask. It has 37000 HP, more than almost any other random encounter enemy in the game, and casts Reflect on itself. It then proceeds to bounce spells such as Bio, Holy, and Flare off of itself and onto the party. In the DS version of the game, this is made even worse by the fact that they now know Laser Barrage, an attack that can hit the entire party for around 2000 damage. Thankfully, unlike the aforementioned Red Dragons, you can cast Stop on them.
Fun thing about Deathmasks and the Dragon pairs? They play the boss music when you fight them. Even the game knows these guys define Boss in Mook Clothing.
Speaking of pairs, the GBA and PSP versions of the game add a twist to the Deathmasks in the Lunar Ruins: They can show up alongside a Behemoth. Against solitary Deathmasks, one can take advantage of their party-wide Reflect-casting to bounce your own attack spells onto it. Doing that with a Behemoth in the picture, however, runs the risk of the spell hitting the beast instead of the mask, which will very likely draw a Maelstrom counter and drop everyone's HP to One, setting you up for a quick finisher from the Deathmask. Hope you like physical attacks and healing with items!
The Gil Turtle. At the time you encounter it, you don't have nearly the XP or skills to defeat it. And it lures you in with a hallway that doubles your gil with each step you take. It has amazing HP, hits very very hard, has poison, is very fast, and makes short work of your party. It'll take quite a bit of reloading and messing around with available skills to defeat this bastard. It's vulnerable to ice and is somewhat undead, so good luck with that! Especially since you'll have to face it again in the Bonus Dungeon.
The standard physical attacks of Mantodea/Greater Mantis will likely do over 3000 damage, killing any character that isn't significantly overleveled. You can first encounter them when your maximum HP is around 2000 or so. They're also found in conjunction with Sprinters, who are pretty benign on their own but who'll cast White Wind and heal the Mantodeas.
Level 20 Magic from the same game. They're only found in a bonus dungeon late in the game, but these guys WILL (not may) pick your party apart. If these buggers appear in twos and manage to cast Banish/X-Zone (depending on which version you're playing), it has a chance to hit everyone in the party with a One-Hit Kill spell. If two party members are already KO'ed by them and the other Level 20 Magic casts it, you better pray it doesn't connect or you'll get Game Over.
The Slam/Veil Dancers in Zozo. When they are alone, they cast Ice2/Blizzara, Fire2/Fira, and Bolt2/Thundara, which will either one-shot a single character or ravage your entire party. In groups, they also carry a move that puts a party member to sleep.
Literally every single damn thing on the Floating Continent. Ninjas love to hit the entire party with powerful elemental attacks and you can't run from encounters including them. The Behemoths have powerful physical attacks as well as Meteo. Dragons can not only use Revenge to dish out crazy damage, but can use Sneeze to remove party members from battle, leaving the remaining two open to a pummeling. Even worse, if you Rasp away their MP to stop Revenge, they start using Tail, which is almost as bad. The Brainpans will Stop multiple party members, leaving them open to tremendous damage from everything else, and one-shot them with Blowfish. Misfits will shred you with Lifeshaver (although you can turn that against them with Gaia Gear). Platinum/Wirey Dragons have high offense and defense, lots of hitpoints, no weak pointsnote except for Relm's Sketch of all things - sketching a Platinum Dragon casts Cyclonic and takes away 90% of their hitpoints, and attack in groups of 3. Apokryphos are usually pretty benign, but Level 4 Flare will one-shot almost anyone who gets hit by it. And the Chest Monster you find early on, Gigantos, hits harder than Ultima Weapon.
Many enemies in Kefka's Tower count as Demonic Spiders. One particular enemy is the Death Machine, which starts by clearing out party members at the wrong levels with Lv. 5 Death, and counterattacks with up to four uses of Blaster in one shot, probably killing anybody who doesn't have a Relic that grants immunity to Instant Death. They also appear with Metal Hitmen, who just plain inflict damage on your party and use Dischord to lower your stats; and Duel Armor, which use Lv. 4 Flare and the aforementioned Lv. 5 Death, as well as Metal Cutter. Then there's the Madam/Outsider combo, which automatically keeps you from healing the entire party while hitting you with powerful elemental ninja attacks and employing the most sophisticated defensive AI in the game.
There's also the Soul Dancers in Jidoor. They throw knives, which ignore both row and defense. Even the lowly Dirk, when Thrown, hits for around 800 damage, and they might even throw a stronger knife, which will put you down real quick.
Everything in the Collapsing House. You're on a six minute time limit to get in, grab the kid, and get out; however, the place is loaded with some crazy powerful items you'll want to snag. However, several of the chests are Chest Monsters, the Scorpions come in groups of three and inflict Condemned status, and Hermit Crabs can inflict Petrify with their final attack.
Evilheads, the bats that accompany Icicles in Gaea's Cliff are faster than you and have two really damaging attacks, but the kicker is that they cheat: they have a 25% chance of setting their Evasion stat to 255 when attacked, making them capable of avoiding even Vincent's perfect-accuracy attacks.
Depending on how well you're equipped, any enemy who can invoke Stop, Stun or instant Death on the protagonist can quickly become one of these during the game's many side missions. Oh, so you didn't have that one accessory equipped? Time to either start over or watch helplessly as your enemies pummel you to death and THEN start over!
Tomahawk Boomerang, Mace Boomerang, or any other cutscene-style attack that's undodgeable. One of Bahamut's forms is particularly aggravating, as it will not only knock a solid chunk of your HP away, but also reduce your MP and AP to 0, so you can't heal or even dodge his next attack.
And then there's the mover type enemies who A) have ridiculously high defense; if you're hitting for, say, 3000 on average, you'll do about 100 to the movers; and B) have the insanely difficult to dodge Delta Attack, which is enough to kill you at a fairly high level.
There are also enemies in the game that are invincible to almost anything short of limit breaks, which happen pretty much randomly. More power if they have life-draining attacks, like G Devastators.
Dispel doesn't sound harmful on its own, until you realise that by the time you encounter enemies who can Dispel you'll almost certainly be needing those (M)Barriers, and that those Dispel-casters can pack a good punch on their own, like those nasty G Paladins.
When mooks start throwing Flare around willy-nilly, it's time to start building towards that HP+ 999% materia.
The worst Demonic Spiders of the game are the Demon Monoliths in the final dungeon on your way to Jecht and the upgraded Behemoths. The Monoliths have Curse, which gives you a crap load of status effects that are basically undodgeable, and the Behemoths cast the powerful Meteor spell when they're killed which will more than likely kill you too unless you have an aeon out. And to top off the Monoliths, they rarely showed up alone. They almost always showed up in pairs or trios. It doesn't help that they possess the attack power of your average endgame boss and always manage to hit with its spammed attacks, which, according to the enemy data, should have incredibly low accuracy. Oh, and to top it off, they can use an attack that inflicts Petrify on your entire party.Have fun.
Then there are the birds in that freaking chocobo race minigame, which can't be classed as an environmental hazard because hazards aren't spiteful. That they swerve to hit you is bad enough. That they add three seconds to your time is awful. That a chocobo dodges like a truck is horrifying, and that they can keep you stunned in place long enough for another bird to smack you in the face is unforgivable. There's a reason no-one likes that minigame.
Anything with the move Curse has the potential to become one of these. Curse inflicts Poison (health drops in chunks at fixed intervals), Sap (health steadily drains at a surprising rate) Confusion (the character attacks themselves or allies) and Disease (Current HP=Max HP, meaning the character cannot be healed until Disease is removed. If a Diseased character dies, when they are rezzed they will have a grand total of 1HP and the Disease status effect still in place, meaning that Disease must be removed and then the character can be healed. Disease cannot be removed with Esuna, the Standard Status Effects remover, but only with the spell Cleanse, the item Vaccine, or the Remedy item, if the player has unlocked that ability for the Remedy). Curse targets all characters. Curse cannot be blocked. Curse will never miss—not even the Malboros' breath attacks are given that luxury. The monsters that wield Curse either come in packs, so that they can make short work of the reeling party before you can rid yourself of the effects, or are giant undead dragons with flunkies who will also use Curse. The only thing to do is to equip your party with Confusion-preventing accessories (you DID remember to buy them, right...?) once you see Curse coming (if you see it in time), have them fling Remedies at each other (you DID remember to upgrade them and buy a ton of them, right...?), and pray that the command-priority-programming is kind to you.
The Shield Wyrms in the Cerobi Steppe don't use cheap tricks like Curse—they will beat the shit out of you legitimately (for the most part). They're a regular enemy, and there are TONS of them in the various Steppe zones—but they have over sixty-thousand HP, more than any other regular monster in the game and more than a fair few of the bosses, marks, and rare monsters too. The HP is complemented by high defense and magic defense stats, and the ability to further augment those stats, and the ability to regain a third of the life bar with an unblockable, virtually unstoppable move called Restore. And offensively? It's a hard hitter with many area-of-effect moves, including the -aga spells and Breath, which can inflict Sap, plus its normal physical can inflict Petrify and combo with frequency. And—this cannot be stressed enough—they're everywhere. Can't throw a stone in Cerobi without hitting (and subsequently aggroing) one of these. And Cerobi Steppe is a biiiiig area. To say it gets aggravating is an understatement. At least they're susceptible to Oiling and Blinding, so you can oil/blind them up and roast them with fire magic, chuckling at their puny defense. They are also susceptible to instant death, which means you can just equip the Nihopaloa and toss a phoenix down at them. Then again, everything is weak to the Nihopaloa.
The goddamn Reavers in Pharos Penumbra. At level sixty they can smack a third of your health bar away and have a nasty habit of walking into battle with Haste and/or Protect (and sometimes Bravery) and a habit of chucking magical Technicks at your party whenever they get low on health. And they always come in groups of three or more. And you need to kill tons of them to advance the dungeon.
The toughest sections of the game are; the sections of The Great Crystal unlocked after getting the Treaty Blade, Pharos Subterra, after beating Cid the 2nd time, and the parts of Henne mines unlocked by the Gariff after acquiring 10 espers. Pretty much everything in those areas are demonic spiders; near-infinitely respawning zombie/skeleton armies, various spirits with the aforementioned Curse ability, Dragons and Behemoths that make up some of the toughest non-bosses around, and downright evil architecture; if it works at all, your minimap is going to be of little use, and if you get lost, chances are you'll run into spiders with depressing regularity. The deepest reaches of Luhsu mines are nearly as bad.
The game allows just about anything that can kill a character from full HP to be a demonic spider. Why? Because if the "lead" character falls, regardless of the other characters, it's game over. So all it takes is for three enemies to "randomly" decide to gang up on the main character, and there's really nothing you can do to prevent reloading. Thankfully, Death Is a Slap on the Wrist.
A lot of monsters in Chapter 11 qualify, unless you've done some serious Level Grinding. The Behemoth Kings seem easy at first, but right before they die they stand up, heal all of their HP and gain powerful area of effect magic, as well as a devastating single-target physical attack.
The infinitely aggravating Vetalas. They put up an 'inertial barrier' at the start of the fight, which reduces all attacks to Scratch Damageat best and doesn't go away until they're staggered - and they stagger excruciatingly slowly. While you grind up the chain gauge, they don't sit idle; they use Multicast to cram powerful magic down your throat and laugh at your puny attacks. And their Palette Swap, Raktavija, is worse in nearly every way, mostly because you can't get a Preemptive Strike even if you use a Deceptisol.
The Scalebeasts in the Sunleth Waterscapes, which are so powerful for this point in the story that the game actively advises you to avoid fighting them - except some of them guard treasure chests with unique accessories. If you can get a Preemptive Strike on them, it makes the fights easier, but they're still much more powerful than anything you've faced.
The Boxed Phalanx and variants thereof. They're tough as nails and hit hard, but that's not the worst. They appear with Hoplites, which up to this point have been fairly weak enemies, and have an ability that can cast every buff in the game on the Hoplites, turning them into Demonic Spiders themselves. They have an attack that hits multiple members of your party. And should you decide to kill off its Hoplites first, it will summon up to five more. They can easily overwhelm a Sentinel's defenses, assuming they don't decide to ignore the sentinel altogether and go after your party leader. Thank God they're vulnerable to status ailments.
The Falco Velocycles that show up in Palumpolum. They have a special move called Gatling Gun that attacks their target with a massive wave of projectiles. Gatling Gun will kill anything with one use that isn't Sentinel Snow.
The Rangdas you encounter in Chapter 11 don't seem too bad, with a serious susceptibility to Fog, but if you leave them alone too long, they use Summon, which tends to bring in a Behemoth King. Have fun with that.
The Sanctum Inquisitrixes, ridiculously fast, resilient, and aggressive enemies you run into in Chapter 12. Not only do they attack fast enough to repeatedly kick you out of your attacks, but they can also cast Bravery and Haste on themselves. Getting Vigilance on yourself or Curse on them will help, but good luck even getting one spell off before they kick your Saboteur or Synergist's teeth in.
The Very Definitely Final Dungeon also has a mini-boss (that also shows up in normal fights shortly after you fight it) called Wladislaus. The fact that it's faster than lightning (much to her annoyance) and hits like a truck is bad enough, but then it unleashes its special attack, Mounting Contempt. This move first inflicts Deprotect and Deshell and removes buffs (focusing first on the ever-useful Haste), THEN piles on enough single-target damage to inflict an easy One-Hit Kill on anything that isn't a Sentinel. One of the hardest fights in the final dungeon is an encounter with a Wladislaus and a Sanctum Templar, which hits hard enough on its own, making it more than capable of finishing off a character that by some miracles survives Mounting Contempt.
Dagonites, found in The Very Definitely Final Dungeon. They start off by using a move that gives them the Vigilance, Haste, Veil, and Enwater buffs, then they start spamming a move called Rush, which knocks whoever gets hit by it down. And they come in packs of three or four, meaning you'll get to watch your characters fly into the air, land, get up, prepare to use an attack, and get knocked in the air again. Rinse and repeat until your whole party is dead. And they also show up with Sacrifices, which will bombard you with Anathema and Death, just in case you feel that you might get the upper hand at any point.
Flowering Cactuars. When you first engage them, they use Parting Dance, which unleashes a massive clusterfuck of -aga spells. If you survive that, they run. And they come in groups of up to five.
Final Fantasy XIII-2 has a bunch. On the bright side, most of them can be caught and used to fight, where they're often just as deadly.
Ispusteke. They're a Palette Swap of Vetala, except with a unique and inexcusably rare drop - the Blaster's Sapphire, which improves a Ravager's chain bonus.
Wladislaus returns, found in the same area as Ispusteke (Archylte Steppe, set to thunderstorm). While it's significantly weaker than its previous incarnation, it still has powerful physical attacks, and Mounting Contempt is still horrifyingly lethal.
Proto-behemoths, found in The Very Definitely Final Dungeon. They appear in one encounter near the end, or they can be summoned by the Mimis. If one of them gets summoned by a Mimi, then you have a swarm of smaller enemies to deal with, alongside the Behemoth. When they appear as an actual encounter, they're flanked by two Schrodingers, who can cast Esuna and Deprotega and drain HP from Serah and Noel, boosting the behemoth's already powerful attacks and making it extremely difficult to kill anything in the fight.
Feral Behemoths in Yaschas Massif 010 AF, when you first arrive. You have to stay inside the spotlights to keep them from running the Mog Clock down, and if you try fighting them, they can easily smash the entire party. And then, in 01X AF, you have the Narasimha, a rare albino Behemoth that is even stronger than the Feral Behemoth. If you have the misfortune of encountering one of those during your first visit, you'd best run.
The toughness of the non-story battles scale with Ramza's level, so if you Level Grind too much you'll find yourself facing some seriously frightening opponents. Bariaus Hill in particular is home to a veritable army of Chocobos, Tauros, Behemoths (who have instant kill abilities), and Dragons.
Some of the rare non-story battles can be an absolute bear. The most talked about of these is the 11-Monk battle on Grog Hill. For those unfamiliar with the game, Monks have the most versatile skill-set in the game, having high attack, a ranged move (that hits multiple people), a 100% hit move, a healing move, a revival move and a move to get rid of status effects, all of which have no charge time. In addition they have decent speed and range and can be pretty dangerous one at a time. Oh, and, they're all wearing gear that makes their ranged move heal each other. They can knock you down and stand themselves back up using the same command. And do. When there are twice as many on the map as you have people in your team at the same or higher level than your highest leveled character, be prepared to die and die horribly.
Chocobos near the beginning of the game can act as Demonic Spiders on their own, having higher attack and movement than any of your beginning classes, a counter attack that you won't be able to get until later on without ample grinding, and healing to boot. Needless to say, running into a pack of these during the first few levels of the game spells instant death. Later on they are demoted to merely Goddamn Bats as they are still annoying, but far outclassed by the stronger classes in the game. However, Red Chocobos and Black Chocobos remain Demonic Spiders until the end of the game, the later having a powerful physical attack, a strong ranged attack and the ability to fly, and the former having a high hitting 100% ranged move with no charge time. Coupled with high speed they can easily take out a healer or wizard before anyone on your team gets a chance to move.
Mandras. They have a projectile special attack which is stronger than their normal attack, and they can still use it from melee range. And if you are in melee, they can use their other special to inflict Sleep — luckily this also affects monsters next to them, but they don't need the backup as they can typically survive two or three hits. Finally, they're a fairly common enemy and show up from an early point in the game, so you will experience them before you have ranged attacks or status defenses.