Bleach: Dark Souls has many unfair fights over the course of the nine times you have to beat story mode to beat it 100%, but there one of the endings to Part C takes the cake. You fight 3 Menos Grande simultaneously, they all start with full spiritual pressure, and they will spam their lasers and teleport attacks.
Large or ranged attacks aren't needed, all you need is a quick, repeatable air attack: due to the combination of their size and the fact that they can still be put into hitstun unlike most oversized boss-type characters, it's easy to perform an infinite combo on them just by spamming the said weak attack while repeatedly jumping towards them. You can also catch all 3 of them into one if you're lucky, and at that point the battle is all but won if you don't screw up the timing too badly.
Aizen in the Turn Back The Pendulum segment of Heat The Soul 6. You can't see any of his info, and one of the characters you're forced to use is one that you've never had the opportunity to use before. He's also one of the most broken characters in that game. Have fun.
Rank S Kisuke. Holy mother of Jesus, RANK S KISUKE. He starts with a full Special bar, his health regenerates (faster during his Power-Up), and he hits like a truck. Also, he has an infinite guard, but he can still shatter your guard in one shot. You're forced to use an Original Soul Code against him, and it's difficult to make a Code that doesn't seal off your abilities while still allowing you to hit harder. And once you've knocked him down, he revives with a quarter of his health. Did I mention that your Special bar will also drain over time?
Preceding Rank S Kisuke is Rank S Mayuri. You again have to use an Original Soul Code, but your opponent has a Soul Code that drains your health over time. It's impossible to make an OSC that allows for health regeneration, so it becomes a Timed Mission. If he uses his Specials and they connect, then you're basically screwed unless you can quickly turn the fight around.
Anybody equipped with any of the DLC Soul Codes become this. There are four, and their effects are: 1) Allowing infinite tag-outs and support attacks (Tech style), 2) Infinite Special bar (Speed style), 3) Two HP bars and more critical hits (Power style), and the grand-daddy of them all: THREE HP bars, ridiculous movement speed, and a full special bar (Speed style).
Soul Calibur III had Hyle, a midboss of one stage in its Story Mode, due to his highly-damaging attacks and a defense so high he was almost invincible. He was actually harder than his stage's end boss. On the other hand, due to how much the computer just plain cheats, every character falls into this category when played by the CPU, down to being literally impossible in the most severe cases.
A few minibosses in Tales of the Sword have the same obscenely broken abilities: they have very high defense and are completely unaffected by attacks. They take damage (slightly), but they won't so much as flinch. This makes them impossible to throw, and they show a frightening tendency to walk you straight off a cliff. The only saving grace is that they take damage every time they attack, so you might be able to stall them out. Maybe. Astaroth fits this too in that mode. If you face him without a new game plus, he can one-hit you with some of his better moves.
Ende. Who the hell is this guy, and why is he killing all of us?
Plus he's fast enough that the only styles powerful enough to do noticeable damage to him get hammered without being able to touch him. Level 61 and 65 Astaroth and Seigfried did maybe a fifth of his health before dying horribly. I hate that guy!
Setsuka, and she's not even a boss (unless she's your character's destined battle).
Kilik can be a nightmare, too, if you had the misfortune of running into him with a slower character. He has a number combos that he can use from a longer distance than most characters that score multiple hits and often either knock you down or launch you. It doesn't help that his stage is a fairly small butte.
The Dancing statue. Large enough that you're the size of its shins. Clearly, can't be rung out or thrown. many of the normal Anti-AI or launcher moves miss completely. It has a ridiculous life bar and speed, lazers, and the kicker? Even if you do beat it, you have to run behind it as fast as you can, cause if it falls on you when it dies, it kills you instantly, before it dies, and you lose.
Kid Buu in Dragon Ball Z Budokai 2 is a phenomenally hard opponent to beat. He has a purple health bar as opposed to green, which the protagonist usually has. For those that don't know, that's about 66.67% more health than our heroes, and it doesn't help if he has the fastest and hardest attacks, opens a can of whoop-ass every time you face him, and after you've beaten him once, it's not over. The sheer formula of the Story Mode makes Kid Buu ridiculously hard to defeat, but it has its rewards once you've beaten it...a new technique.
Plus, Buu is half the size of the adult-sized characters, so everyone punches and kicks over his head most of the time, meaning that you have to resort to Beam Spamming from a distance, and even then it's still hard to keep him away from you considering how fast he is.
Budokai Tenkaichi 2 has Android 18. She has two specials: Infinity Bullet (an incredibly annoying Beam Spam) and Destructo Disc (a double beam attack that is impossible to block). Most of the times you face her it is immediately after facing Android 17, who is notorious for draining your health levels to near-nil. One of the few times you don't, however, she goes from "incredibly annoying" to "snap the game disc in half". You face her with Hercule, in an arena that is an instant loss if you fall outside a very small space. Hercule cannot fly on his own. If his jetpack runs out of power, he will fall. You pretty much have to pray to the Random Number God that she doesn't just decide to fly outside the ring and start spamming Infinity Bullet. Or that she doesn't rush you right at the start before you can use False Courage (which makes Hercule more of a Lethal Joke Character). Or that she doesn't run outside the ring as you attack. Or that she doesn't Destructo Disc you before you attack. Or that False Courage doesn't wear off. Even if your health is full and hers is almost empty, you can still lose very easily and very quickly with the tiniest bit of bad luck.
And woe betide you if you're after 100% Completion, as it requires doing this particular fight twice. And just when you think it's over, you face Krillin, of all people, with the exact same character in the exact same arena - it's very nearly as much of a Luck-Based Mission.
Also in Budokai Tenkaichi 2 is Kid Buu, mostly due to the fact his first course of action will be to use his Rush attack, which will launch you across the arena, and it takes a while to get up... Which is enough time for him to charge it up and use it over and over, worse is that, unlike most Rush attacks, this one actually travels a fairly impressive distance.
Budokai 3 has, if you set the difficulty on Z3, pretty much every boss ever on Dragon Universe. If you're starting a new character at level one are and trying to get him to level up faster, you will die in a matter of seconds. Basically, while you have three bars of health, the very first enemy of every DU story has seven bars of health,which is more than 233% of your health. If that isn't enough, he has about as high defense and attack power and can teleport out of anything. Keep in mind that I'm talking about the first enemy.
Soul Calibur IV has, somewhat expectantly, the Star Wars characters. Particularly ridiculous, as this latest entry in the series has introduced Algol, supposedly the most powerful character in the entire series, notable for dual-wielding the titular MacGuffinand its evil counterpart. Yet in Arcade Mode, it will be the lightsaber-wielders who will force the untrained to hit Retry seven million times. Guess Star Wars characters in the 16th century, and lightsabers in general, are just that awesome?
Arguably out of the two, Yoda is the bigger "That One Boss", since he's so small, all High attacks, and some Mid attacks will go right over his head! And you can't throw him. At all. But he can throw you, oh yes. He is, however, easier to ring out than usual.
But then again, The Apprentice plays perfectly. He will always have his block in place for any type of attack, sometimes even while he is still on the floor (your attack propels him to stand with his guard up). He has several unblockable moves which are all executed faster than normal. He breaks your throws with ridiculous ease, yet you can't seem to break out of his. It gets to the point where by the time you reach Algol, you will probably be thinking, 'At last I can breathe easy'.
Think that's bad? Try doing floors 57-59 of the Tower of Souls mode, which when taken as a whole (the three floors must be cleared one after another as part of a single mission) could easily qualify as That One Boss. Let's do a quick run-down: you are faced with 3 tag-team battles with 3 opponents in each one, lose any battle and it's back to the first with you. All 9 opponents are set to the highest difficulty (so they'll be pulling off insane combos and dodging/countering your attacks with superhuman precision) and feature a wide variety of fighting styles (fast, slow, close-range, long-range, etc). You, on the other hand, can only take 2 characters in with you and neither can regain health between each battle (aside from the slow health-regeneration you get when a character is tagged out, but your opponents get that too). The first battle takes place in a wall-less circle, so expect to see one (or both!) of your characters ringed-out right when you're on the verge of winning. The second battle takes place in a thankfully enclosed arena, but that doesn't stop the computer from juggling you against the wall. The third and final is another open-air fight, but by then your characters will likely have so little health left (if you even made it that far!) that they won't even live long enough to get pushed to the edge. Not enough for you? The computer abuses the HELL out of the tag-team mechanic, enabling your opponents to pull off juggle combos and stun-locks with all three jumping in and out like crazy that would be impossible for any individual fighter to do. That last fact alone makes 57-59 a NIGHTMARE (who, conveniently, is the boss of that mission) It's perhaps telling of the absurdity of this mission that the one immediately after it, Floor 60, which pits you against a ridiculously-overpowered version of the aforementioned Algol, is EASIER.
Floor 20 is just terrible, since you have to face a Taki knockoff with abilities that make a good percentage of her guards impacts, which damage you and heal her. Oh, and you can't throw her, so good luck pulling off an Unblockable in time.
The floor "King of The Physical Realm" is complete bull. Every character's defense is at maximum, meaning they take the same amount of damage as they would if they blocked an attack (which is less than a milimeter off their health bar). Not only that, they sidestep, counter, throw, and are beefed up with those nasty, unobtainable "S" abilities. Luckily, these floors can be conquered easily if the player is playing as Seigfried and spam his crouching A+X. The computer makes no attempt to block this attack and it does normal damage, despite their maximum defense.
Soul Calibur V doesn't get quite as bad as some of the more ridiculous examples from previous entries, but you will be hard pressed to find anyone that considers the fight against Nightmare at the end of Arcade Mode anything more than an extremely frustrating chore. Most of Arcade mode is pretty simple if you're playing on the difficulty setting that suits your skill level, but even on easy, Boss Nightmare will fuck you up with his his incredibly aggressive AI, perfect spacing so that he's almost always in optimal range to clobber you without getting hit, and a tendency to use his Critical Edge to counter your attack, smash two thirds of your health bar into oblivion, and then kill you before you can get up again. On harder difficulties, letting Nightmare knock you down even once with his big sweeping attacks can be a death sentence. And then, of course, if that boss was too easy for you, there's Legendary Souls mode, where every character you fight is set to boss mode, making them a Perfect Play A.I. that reads your controllers and will tend to walk all over your every attempt to attack. And you have to beat our old friend Algol at the end of this to unlock him.
If there's anyone else who spent long days wishing that Gill from Street Fighter III would go directly to hell without passing go, know that you're not alone.
While we're on the topic of Street Fighter, one would think that Alpha 3's Final Boss of M. Bison was enough of an utter bastard to deal with. Warping all around the screen, air throwing you without warning, and woe be to anyone caught in mid-air during a Psycho Crusher that wipes out half your health.
Street Fighter IV. SSEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEETTTTTHHHHH!!!! Where to begin...? Sure, Seth fights fairly in the first round, but you KO him once, and he goes insane. Teleports, triple-hit Shoryukens, an attack that sucks you in towards him and stuns you at the same time, no-lag Zangief grapples, his annoying grabs, Sonic Booms, a super that drags you in and is unblockable, and an ultra that is unblockable and almost unavoidable... And this is just on easy mode. It's not uncommon for Seth to wipe out half your health within five seconds of the round starting. If you're playing with one round matches instead of "best of," he doesn't even give you that first round of fair fighting.
Believe it or not, Seth is actually worse than described above. Sure, his ultra is actually blockable, and his super isn't that bad because he'll be using EX moves like they're going out of style, but what really makes him annoying is that he ignores the difficulty level. If you're playing on Easiest or Very Easy with a character you're not entirely comfortable with, he'll massacre you. Even on Easiest, by definition the easiest difficulty level, he can jump into the middle of an active ultra combo execution (for example, Rufus' Space Opera Symphony, which has a half-second delay between hit two and three) and either smash you with a four-hit combo or just use the piledriver. And on Hardest difficulty, the aforementioned first round of fair fighting disappears, and he's insane for all three potential rounds (or more!). The only good news is that if you're able to consistently beat him on Easiest, you're able to consistently beat him on Hardest as well, since, as mentioned previously, he ignores the difficulty level.
Seth is easy if you realize he has an easy to exploit AI loop with him where you get a half screen away from him, jump towards him, jump kick, followed by a low kick. Jump back and repeat the loop.
Seth is no different in Super SF 4, still being as annoying as the last game. The main difference now is half the time he'll run his second ultra, which under normal circumstances is less annoying than his original ultra because it isn't unblockable... but it does make you think twice about using aerial attacks instead. And if you just read about his AI Loop...
How about Sagat? Fight him in Street Fighter II. He will use a <del>my turn</del> Tiger attack every time. Ryu and Ken are also hard to deal with thanks to their Hadoken and Shoryuken spamming. He might even be worse in the Alpha games. At least his voice is better in that game...
That's him demoted from Final Boss (in the first game, beating anyone is quite possibly a Luck-Based Mission; pulling off special moves is harrowingly difficult, but they are almost necessary to win).
Tabuu from Super Smash Bros. Brawl. He features a smash attack that effectively delivers a one hit knockout to the entire screen. It's nearly impossible to recover from. Not so much as the final boss in adventure mode with six 'lives' to use against him, but in Boss Rush, where it's only one chance.
Tabuu's cheap attack can only be dodged by rolling (or sidestepping, or air-dodging) at exactly the right time. Three times in quick succession. Thankfully, at least on the lower difficulties in the Boss Rush mode, these attacks are toned down considerably. Fail to dodge one in the higher difficulties and you have to start all the way from the beginning.
Don't forget Porky/Pokey, who not only has crazy damaging lasers and fatty-bombers, but hurts you by merely WALKING INTO YOU. And when he starts running, which he does without warning, if he so much as touches you, you will most likely be sent flying off-screen.
Porky does a small jump and vibration before rushing across the screen like nuts. Enough for someone with quick reflexes to get away. Also, has a jump attack that really does have no warning, and said crazy damaging lasers are tracking lasers, making them almost impossible to avoid.
Rayquaza, considering that he hits very hard and very fast, and this only gets worse when you damage him enough and he Turns Red, causing him to become even more vicious, as he now hits harder and moves faster than before. His attacks are all tricky to dodge, but his Thunder attack takes the cake, as it's nearly impossible to evade, and must be blocked with such precise timing that if you don't, it is guaranteed to hit you For Massive Damage while you are dizzy from the attack, allowing Rayquaza to follow through with another free attack.
Giant Shadow Bug Diddy Kong. Takes up nearly the entire screen, hits like a goddamn elephant, and is every bit as fast as regular Diddy. Add in the fact that when you first have to fight him, you only have three of the lighter characters in the game. Not only is Giant Shadow Bug Diddy Kong more powerful than the game should reasonably allow for a mini-boss, but you have to deal with it by pushing it off the screen like regular fighters. For references' sake, most bosses die when you merely get their HP down to zero, but for Giant Shadow Bug Diddy Kong you have to get its percentage really high before you can even think of KOing it. Add in all that speed, power, and weight, and you have a guy that makes short work of your fighters before you can get close to KOing it.
Not to mention that you fight him on a stage with absolutely NO PITS whatsoever. In the Super Smash Bros. games, normally giant character fights are offset by the ease with which they're thrown down bottomless trenches. And unlike other boss characters, he can also pick up items which SCALE TO HIS GIANT SIZE. Think you've got the bastard on the ropes? He picks up a beam sword which then promptly allows him to hit half of the screen with one swing.
Oh and Giant Shadow Bug Diddy can occasionally KO you in one hit depending on the level
Best PRAY he doesn't even think of using his Smash Down-A move, or you're dead. Seriously, who at Nintendo even fought this guy during development?
To make matters even worse, Diddy likes jump the gun sometimes, meaning he'll often get a free hit in on you before you're able to react. Watch here as he scores a double K.O. literally the second the match starts. And this is on Normal mode...
To say absolutely nothing of Duon, the two-sided giant robot of DEATH. Both halves will rain bombs, lasers, and missiles at you when it's not pounding you flat into the ground — and it still manages to do all that faster than you can blink. Pure, unadulterated hell!
One nice thing about Duon, though, is that if you're using Lucas or Ness, one of his attacks, the pink projectiles he fires will heal you if you're using Psi Magnet (Down+B). On Boss Rush mode, getting Duon near the end was like a free heart. (Master Hand's bullets work the same way.)
Meta-Ridley takes an odd position here... when you first meet him in Adventure Mode, he's not that big of a deal, but his appearance in Boss Rush Mode (Where there is no such thing as 'Continues') is quite dangerous, due to the dangerous stage you fight on, and his ability to temporarily remove the ground under your feet... it doesn't take a very big mistake to get you KO'd against him.
Speaking of Meta-Ridley, attempting to capture him with a trophy stand is an absolute nightmare. First off, he's at the end of one of the longest, most frustrating stages in the whole game, meaning that if you accidentally defeat him, you have to go all the way back to the beginning (Considering that lowering his health is an obligation, that just adds to the frustration). Second, he has two moves (one where he stomps the platform and another where he drags it down) that move the platform below the bottom blast line, eliminating any trophy stands that may be on the platform. Third, there's a two-minute time limit, meaning that if you take too long, you have to restart the battle and you have to lower his health all over again (at least you don't have to restart the whole level). Fourth, he can easily accidentally kill himself with a bomb or some other item that appears on the stage, once again forcing you to restart the level. Fifth, 95% of the time, he's flying over an abyss, meaning that even if you do manage to snag him, the trophy stand can fall into the pit (unless you manage to catch it, which is FAR easier said than done). Catching him doubles as a kill, again, forcing you to restart, and even when he's not above the abyss, the bastard shoots lots of crap and won't stay still. If you actually manage to obtain his trophy, you deserve a real trophy.
Ness in the original SSB can be surprisingly difficult the first time you fight him, due his increased AI when fighting him compared to the other 3 unlockable characters and his annoying projectile attacks.
The Showdown event of Super Smash Bros. Melee was pretty brutal and the hardest bosses (Giga Bowser, Mewtwo, and Ganondorf) of that game without question.
Super Smash Bros. Brawl repeats this with the FINAL final battle event, consisting of Sonic, Snake, and a Giga Bowser-sized Mario, which uses a completely different strategy but is basically the same idea.
The Master Hand and Crazy Hand team up fight at the end of Classic mode in Brawl can be really difficult. First off, unlike in Melee, you can no longer fight against the two on Normal mode - they have to be fought on hard difficulty or higher, but you still have to make it to the last fight under nine minutes without continuing. In Melee, after you damage Master Hand enough, Crazy Hand appears at full health; in Brawl, both appear at the start of the fight. Both hands have roughly the same attacks, but while both are alive, they have several combo attacks that will all but one-shot anyone they hit.
Master Hand and Crazy Hand can turn into this in Melee as a result of Fake Difficulty. Simple rule with Classic Mode's difficulty setting, the higher you set it, the more HP they have and the more frequently they attack. The problem is that in Melee's Classic Mode, you can't turn the timer off, and if it runs out, you lose automatically regardless of how many lives you have. Where this gets to is that even if you get good at dodging their attacks, the fact the sheer rate at which Crazy Hand alone attacks even on normal means that it's entirely likely that you'll lose just to running out of time as opposed to either of them killing you.
Metal Mario from the original SSB. Good lord, where to begin? For one thing, he's even heavier than Giant DK (and a much smaller target to boot), so none of your attacks will even make him flinch. This makes fighting him at close range a losing prospect, since he can hit you a lot harder than you can hit him. You can try to keep your distance and hit him from afar, but even that's made difficult by the fact that you have to fight him on a tiny stage where there's barely any room to maneuver (and God help you if you chose DK or Captain Falcon, who don't have any ranged attacks). Granted, his massive weight makes it near- impossible for him to recover after being knocked off the stage, but you have to get his damage up to 150% at least before you can even dream of knocking him off. Hell, you need to get some serious damage in before you can even knock him down.
Geese Howard is usually the toughest boss in every game that has him, especially the Real Bout Fatal Fury games, where his regular attacks are beefed up.
Raiden the pro wrestler of the original Fatal Fury could be considered That One Boss as well. Try to fight him at close range, and he'll grab you and choke you to death. Keep your distance, and he'll blast you with his breath attack.
Goenitz of The King of Fighters 96 and by proxy, 98UM, is considered to be the most annoying boss in the entire damn series, with XI's Magaki and 2001's Igniz second and third, respectively. What makes the 'Nitz so irritating is the fact that his wind pillar special will usually land right on top of you, forcing you to block, if not nullifying your projectile to prevent a fireball spam victory. It's for this same reason that jump-ins are impossible to do. On top of that, he doesn't even need a full super meter to pull of his powerful grab super: if that doesn't kill you right off, the wind pillar probably will.
Magaki, meanwhile, spams blue orbs that knock off a fourth of your health, pink orbs that can appear from anywhere on the screen, and a super that will hit you - it's full-screen.
The pink orbs have some specifics: There are ones that can be fired and teleport, and there are the annoying ones that come out instantaneously, acting as a wall in front of Magaki... which tends to be used as you approach and strike. And as for the blue orbs, they don't disappear until they leave the screen. What's worse, Magaki can have two or three blue spheres moving in their predetermined paths AND perform other attacks afterwards, which tends to make the area downright congested.
Any and ALL incarnations of Rugal. If 94 Rugal isn't Genocide Cuttering you into pieces, 95 Omega Rugal is abusing 95's already Nintendo Hard AI. If 98 Omega Rugal isn't having fun with Vanishing Rush and Gravity Smash, then 2002 Rugal is pinning you down with Kaiser Waves that come out in a mere milli second. You should be thankful his MAX 2 isn't very useful...except in 2002UM, wherein it can OTG you. Yay.
Mildred Avalon in Arcana Heart's Story Mode. As the final boss, she's capable of using every single Arcana super attack as her suite of regular attacks, and has an incredibly difficult-to-block screen-filling Limit Break that can deplete a large chunk of your life bar with one use. What's worse is that in the first phase of this battle, she hides inside a cocoon with two laser-shooting Attack Drones that must be defeated in one round. Sounds easy at first, but if you fail to beat her cocoon form, Millie's Super Meter is maxed out at the beginning of her next phase, allowing her to fire off her Limit Break attack almost at will.
And yet she is impossibly less difficult than Parace L'Sia of the sequels. She fires projectiles with reckless abandon, has absolute priority on all of her moves, regenerates damage, and most of her supers are a One-Hit Kill! One of her supers is a projectile that goes off five seconds after it hits you, suddenly dealing insane damage and putting you into hit-stun - even when you're in the middle of a combo or blocking a multi-hit attack. Just to make things worse, she continuously regenerates her super meter, and has another move where she goes into a cocoon to accelerate its regeneration. The cocoon move only ends when her super meter is completely filled (at nine stocks) or if you attack the cocoon enough times to break her out of it - and none of your attacks will do any damage during this time. And she's still regenerating her health. She goes well beyond a typical SNK boss and acts more like a overpowered horrible MUGEN character.
Arcana Heart 2 gives us the somewhat less frustrating Angelia. In arcade mode, she simply plays like a smart computer opponent (including her annoyingly effective but technically sound "Halo Dash->Halo Super" tactic) and doesn't seem to use her arcana at all... but she still doesn't seem to have charge her halo dash attack, since she can and does use it constantly (especially during attack clashes). In Story mode, on the other hand, she uses the same tactics, but her arcana (Mildred, incidentally) is always active, meaning Mildred launches attacks on her own pretty much independently. In addition Angelia also has a certain unavailable-to-players super where she... drops a building on you. Seriously. Needless to say, it's unblockable and does over half a health bar of damage, though she thankfully only uses it infrequently.
In Sugoi! Arcana Heart 2, players can use Angelia's building drop super.
Arcana Heart 3 tops it off with first having you fight mid-boss Scharlachrot, who plays like Hazama with robot tigers that fire beam spam cannons. To wit, she attacks really aggressively, her Arcana (said robot tiger) can interrupt your combo by shooting you in the middle of it and set her up to combo you (and it will happen often). One of the Arcana supers are to fire a super powerful bullet capable of wiping out nearly 50% of your health, and her Arcana Blaze is to fire a frigging huge beam that goes upwards and will take out 50% even in chip damage. Oh, and beat her on the first round and she goes to activate PERMANENT Super Mode with boosted attack and defense. And did we mention that her Arcana bar usually regenerates in an unusually fast rate so you'll find yourself often facing that near-50% health powerful bullet? Win against her? Then prepare to fight Ragnarok, a Humongous Mecha bent on destroying Japan and plays differently from the rest of the game on a 3-tiered battlefield - if you do not understand how to fight bosses like Onslaught, you're pretty much done for, and his main offense are two annoying flying funnels that constantly fly around you and interrupt your acts, not to mention Bullet Hell gallore or a beam rain unblockable. Much like Mildred, this is Do Or Die - run out of time and Japan gets blown sky-high. In short, the final boss is essentially made (overly) difficult not by its moves or its strategy, but by the fact that it basically converts the game into a side-scrolling platformer, while you're still saddled with the control scheme of a fighting game. If the spam attacks don't get you, the clock will simply run out while you're trying to jump up and hit that last weak point.
EVERY SINGLE DAMN BOSS in Neo Geo Battle Coliseum. Mizuchi is Orochi, inheriting Goenitz's gratuitous pillar spam habits, as he retains Orochi's Lightning Pillar move that hits at random spots. Only now he abuses it like a motherfucker, and can throw TWO of them out at any time, not to mention an unblockable grab super and another projectile that reflects your own projectiles. NEO-DIO has a counter that causes him to teleport behind you and respond in any way he desires, and he's a total block whore. Shishioh/King Lion has a crazy amount of range and a counter that he can combo off of. Whats more, THAT version of King Lion, called King Leo, is not usable, leaving you with a Nerfed King Lion to fight with. Goodman...well, when you consider that he puts all these other bosses to shame, the less said, the better.
In Def Jam: Fight for New York, Crack easily qualifies. He can take much more punishment than any other opponent before him, and can counter like no one else. Not to mention the fight with him and Magic later on, with a heavily battered team-mate. ARGH!
Protip: As painful as it sounds, on that battle, you have to take Crack out first. Magic is a Submissions fighter, and although Blaze has half health, his limbs are at full strength. Blaze can last quite a while against him. Crack, on the other hand, is both a Wrestler and Street Fighter, and thus has two of the easiest ways to knock someone out.
Protip 2: Even though normally Def Jam rewards you (to an insane level) for favoring offense over defense, by far the easiest way to beat Crack is to block his punches, then counter when he when he goes for a grapple. (He's easy to grapple when he's recovering from this). If he does his leaping street fighter punch, just block, then either hit him back or grapple him.
Marvel vs. Capcom: Clash of Super Heroes: Onslaught, Know His Name and Fear it. Simply because he is so much more difficult than anything else in the game. Needless to say he also comes with multiple forms and screen-filling laser blasts.
Actually not as bad as many other examples on this page but definitely a qualifier for the one boss of the Marvel vs Games. Magneto from Children of the Atom could take the title too though. Note they compete for that one boss of the entire Marvel series, as final bosses they don't count for the game.
Well since we're on the Capcom Vs Series, Cyber Akuma. Spams the warp move, blocks at every single attack I throw at him, spams projectiles and has his speed set to hyper. Argued to at least be worse than Onslaught.
Abyss, namely the second and third form. The second form is a pain if you don't have a decent projectile user as he spams a pushback flame cannon follow by a Wave Motion Gun, while the third disappears constantly and tends to appear and attack offscreen.
Motaro in Mortal Kombat 3. Both his punch and kick alone can shave off a good quarter of your health and send you flying across the screen, invokes Teleport Spam and usually grabs when coming out of it. His sweep involves using his tail and it sweeps the entire floor and he can fire his damaging projectile attack in very quick succession, something you can't do while playing as him. Oh, and all projectiles bounce off of him and the Spear attacks go right through him. He's somehow even MORE broken in the N64 version of Trilogy as a playable character where you can actually kick WHILE MOVING.
Kintaro as well, to the point where gamers were pleading to Midway for help with him, since he's like a bigger, faster, and stronger version of Goro from the first game. Punches and kicks like a goddamn tank, his uppercut knocks you off screen, his projectile knocks you down, he can grab you while you're in midair, he has incredible range with both his basic attacks and his special attacks like his fireball which he loves to spam, and he's also the originator of the Teleport Stomp. Ouch.
The MK Trilogy had Noob Saibot as a playable Game Breaker character, but The Elder Gods help you if you actually fought him on any level higher than "VERY Easy"...
Beat Motaro and you get the even worse Shao Kahn, but at least he's a final boss, so it was to be expected.
In Mortal Kombat Deadly Alliance, all the hype surrounded the terrible worlds-threatening alliance between Quan Chi and Shang Tsung, and this duo is indeed the last fight (you have to win one round against each). The second to last fight? Some barely-mentioned demon called Moloch...who hits like a tank, takes far less damage than anyone else, can throw you out of anything, and even has a guard break. (Onaga in Deception was pretty similar, but not nearly as brutal, and there's no one after him.)
Shao Kahn in Mortal Kombat 2011 is a nightmare. He can rapid-fire spears, throw his hammer which is unblockable and stuns you, abuses his ability to pretty much negate the hitstun of your attacks to perform a counterattack, takes 50% less damage than you do, does more damage than you, and if he hits you with his X-Ray move, you might as well just let him finish you off. and try again next round.
Alpha from Super Cosplay War Ultra is a brutal combatant, definitely up there with most SNK bosses. An almost-screen-filling projectile move he loves to spam, a ridiculously effective command throw that he can use to interrupt slightly-slow combos that can cancel into any of his supers, aforementioned supers that can very easily break your character in half, and a maximum of 5 super levels instead of 3, starting off with 4 levels instead of 2. He can very very tough to beat, if you don't know how to use your character's tricks to their fullest extent and understand Alpha's strategies enough to be able to circumvent them.
Also, Zenka. She also has the 5 super thing, the three-quarters-screen-hitting specials (not even supers), and a three-level super that is unblockable and almost completely undodgeable. Hatred.
Neko-arc (a friggin Joke Character), the boss-before-The Dragon. She is ridiculously overpowered for a Joke Character, has both short and long-ranged moves, and is too small for most long-range attacks to hit properly.
G-Akiha (Akiha turned mansion-sized). She is so giant that only her upper body shows, which makes her completely impossible to throw, trip, or stop in any way, while her attacks do ridiculous damage. As combos and specials are all but unusable, strategies generally stick to hiding in the corner and sniping, or (with certain non-ranged characters or in story mode) dashing forward to quickly peck at her health, then retreating as fast as possible. Rinse and repeat.
In the new installment Actress Again, we have the final boss of Arcade mode "Dust of Osiris" which is basically an alternate version ofSion on a giant robo. Its a joke in arcade mode but in Boss Rush mode? Its heat gauge constantly fills until it can unleash its unblockable, undodgeable arc drive, usually resulting in death. Good luck to anyone using Akiha Vermilion!!
What makes Tengu even harder to beat is the fact that the screen is all blury and the only way to fix it is to go nuclear on his ass.
Trying to beat ALPHA-152 with the big characters (Bass, Bayman, Leon) is like, as the AVGN said, picking fly shit out of pepper, WITH BOXING GLOVES!
Curfue from Asura Blade. The game steals a LOT from both Guilty Gear and SNK. Curfue, however, steals from Capcom. He's essentially Cable with a few of Justice's moves, while the final boss, Geist, is Justice with a few sucky charges and NO beam cannon. However, note that he has many anti-air moves so it's a straight-on slugfest. Curfue will simply fill the screen with beamspams and grenades and cataclysmic explosions. Much like any other beam spammer in a fighting game, merely blocking the beams incurs serious damage. His throw involves him bayonetting you and then firing a fully charged blast at point-blank range. To a lesser extent, playable character Talos is somewhere between this and SNKBoss as he does ridiculous amounts of damage and has a huge range, due to him being pretty much a medieval version of the Marvel fighters' Sentinel, however unlike SNK Bosses he has a long window for larger moves (but when they go off, they go off FAST!). In fact his normal fierce punch does about 70% damage!
Jinpachi Mishima, from Tekken 5 marks the series' proud debut into this trope. Unblockable high priority stun move? Check. Unblockable fireball when no-one else has one? Check. Use the fireball anything up to seven times in quick succession if the opponent has the temerity to sidestep it? Check. Controller through TV and cat hiding under the sofa? Check.
Azazel (Ogre meets Jinpachi meets Devil) and NANCY-MI 847 J from Tekken 6 are even worse.
To put it in perspective the one thing that makes Azazel hard is not offence but DEFENSE. He can't be damaged if he's not attacking or on the ground. And when he is attacking he is spamming combos constantly. Fortunately most can be blocked for no damage unless he puts his fist into the ground and those stupid pillars pop up.Nancy has several unblockable attacks (nearly all of her attacks are) but can also TAKE THE FLOOR OUT FROM UNDER YOU FOR A RING OUT KO!
Screw Alisa in scenario, that backstabbing WHORE. If she goes away too far, BAM! healing taunt. Get too close, and she breaks out the chainsaws, which have a high rate of freeze. That's annoying, but it gets compounded by the horrible camera that would make the camera in the new Ninja Gaiden games look good. And the controls are just plain terrible.
Also, from a historic viewpoint, Heihachi and Kazuya in T2 always seemed tougher than Devil by a mile. The worst part was that you couldn't really punish certain moves like you can now (low parrying didn't exist, period, and Kazuya was prone to random tenstrings at the time when such a thing would destroy you, while Devil was prone to random flying lasers that allowed you to run under him and grab him from behind, usually turning the tables significantly).
Mike Tyson/Mr. Dream from the NES classic Punch-Out!! So Little Mac works his way through a hoard of, for the most part, fairly easy boxers (except in the last circuit, where it gets tough, but not terribly so) just to end up in the "dream match" with one chance to beat the legend or be forced to retire. Seems simple enough... memorize his pattern of attack and strike back accordingly right? Well... not so right, Tyson/Dream will knock you out with 1 single punch... not to mention that seems to be his FAVORITE punch so you will repeatedly be knocked down with 1 hit just by failing to dodge... he is possible, just keep your wits about you and dodge every. single. blow. Good luck, I've been working on that one for ages.
He only uses the knockout blow for the first 90 seconds, or about 30 seconds of real time. The other uppercuts do less damage after that. If you can get past that without getting knocked down, you've got a fair shot at him. Though even after that he's still mad fast. Every boxer in the game has some sort of pattern to him, Mike Tyson is no exception.
In the Wii version, it's very likely that you'll hit a wall in the first World Circuit match against Aran Ryan (last seen in Super Punch-Out!! and taken a few levels in "Angry Irishman"), as he stands still a lot less than other opponents, making it very hard (if not impossible) to land combos and Star Punches.
Also in the Wii version is Title Defense mode, which more or less turns every opponent into That One Boss. Special mention, however, must go to Title Defense Bald Bull, who, in addition to having merciless timing and powerful attacks, can't be knocked down with a normal punch, requiring you to Star Punch him just to knock him down.
He was the same way in the original game when you fought him a second time in the World Circuit.
But you could also knock him down by countering the Bull Charge, and, believe it or not, he had less staying power than the first time. In the Wii game, not only doesn't countering the Bull Charge knock him down, he always gets up at full health.
Hitting him with a 3-star punch after he walks back from the Bull Charge will always grant you an instant KO, but good luck getting 3 stars and lasting that long.
What really elevates Bald Bull to That One Boss status is that once you beat him...on Contender or Title Defense...you should have no trouble at all taking out Super Macho Man and Mr. Sandman. (The former falls easily to good reflexes; the latter is pure rote memorization.)
Soda Popinski in the original game, where not only does he have an entirely different way of fightingnote he's a lefty, while all of the other boxers are right-handed, but you also begin with very few hearts, meaning that if you make too many mistakes, you'll be left tired and be even less able to dodge his attacks.
Soda Popinski is also responsible for That One Achievement, in which you must TKO him within an incredibly short amount of time. It is even worse when you consider that some of the walkthroughs for that particular achievement are WRONG.
unknown from Eternal Fighter Zero Memorial. Her attacks have priority over practically everything, and she has a tendency to spam the screen with giant orbs of death. And you can't recoil guard against her. At all.
Don't forget Kanna, who not only deals bucketloads of damage, but has absolutely insane zoning and can combo you into tomorrow, and attacks with crazy priority, including an attack that homes in on you a super move that fills the screen. Fun times for all!
Ruru in Magical Battle Arena, who hits fast, hits hard, and constantly interrupts your attacks if you're within her range. The general advice against her is to run like hell and spam Super Moves at her by taking out the surrounding Gadget Drones to fill your Charge Meter.
And it gets worse in Nowel's scenario where, for her final stage, Ruru is accompanied by non-handicapped versions of Kirara, Sarara,Nanoha, and Fate. And you thought being peppered by Gadget Drone shots was bad...
Any time you have to fight Orochimaru in the story mode in Naruto: Ultimate Ninja 2 is tough, but the clincher was the first time you fight him, as The Third Hokage. It wouldn't be so bad if you weren't required to finish him off using a level 3 Secret Technique, which Orochimaru simply WILL NOT let you do. He blocks every head-on attempt at it, attacks you if he catches you charging up for it, and if you try to run away from him and charge up, he runs after you and smacks you before you get a chance to use it. And because a level 3 Secret Technique requires full Chakra, and getting knocked down in this game makes you lose Chakra, you spend most of the battle running around trying to get your Chakra full, all the while getting the crap kicked out of you by Orochimaru. Even using the Substitution Jutsu to get behind Orochimaru doesn't work, as that itself causes you to lose Chakra, meaning that even if you do get behind him, you won't have enough Chakra to use the move anyway. Plus, the initial attack that has to connect in order for the move to work is so slow that Orochimaru usually has plenty of time to notice you and block/counterattack no matter what you do. It's always been a matter of luck for this troper. Not to mention he really likes using this move called "Get out of my face!" which is an unavoidable (after the initial attack connects) move that sends your character flying. And since this is a stage where you can fall off the edge... It only makes you take a little damage, but it gives Orochimaru a chance to "camp" your respawn point and attack you as soon as you reappear. A chance which he often takes.
In that game we also have Deidara during Sasuke vs. Deidara round 2 from the story mode. He spends the entire fight riding on C2 Dragon, making him near impossible to attack should you get close, which is a feat in itself due to his constantly spamming his bombs from the opposite side of the battlefield. Hell, the only way to reliably attack him is with your fireball, which is difficult due to the aforementioned spamming stopping you as you attack. Regular Deidara is only 'slightly' better due to at least being able to attack him if you get close enough.
And then there's Evil!Sage Mode Naruto. His version of Rasengan, RasenganBarrage, is even faster than a normal Rasengan or Chidori and is perhaps the fastest jutsu in the game. His Ultimate Jutsu, a thrown Futon: Rasenshuriken, cuts through almost everything in the game and curves slightly. His chakra shuriken is not one, but two Fuma Shuriken that beat other chakra shuriken and deal good damage for shuriken. His grab, which also have cutscenes, sends out a clone that, like most grabs, have ridiculous priority; if you do hit him, you still might be grabbed. Oh yeah, if by some display of serious skill or sheer luck, you get his health down low enough, he can activate his Awakening. His awakening is his Six-Tailed Mini-Kyuubi form. He hits hard. His jutsu is now the Chakra Gun Volley; three rapid fire guard-breaking chakra shotsthat can hit you while you're on the ground. It easily can be timed to hit you as soon as you get back on your feet. Good luck. He also has a permanent boost that increases his defense, making your attacks hit for less damage.
Taka!Sasuke against Hachibi!Killer Bee. If you even consider using melee attacks, he will hurt you. Bad. Which means that your only method of attack is chakra shuriken and, if you run out of chakra, normal shuriken, both of which do miniscule damage due to him being Made of Iron. You could load chakra, if only he wouldn't spam his own distance attack, a blast of air that does a fair amount of damage and can be spammed when you're trying to get up. Ironically, if you manage to enter Awakening Mode, he becomes fairly easy to bet by spamming Amaterasu, which he almost never blocks and usually takes about 10% of his life bar. That said, actually entering Awakening Mode can be rather difficult as it requires you to not only have about 40-45% health left (yes, the battle requires you to lose health in order to win), have a full chakra bar, but also stand perfectly still and charge chakra for several seconds, making you a sitting duck for his spamming of aforementioned air blasts.
In Naruto: Clash of Ninja Revolution 2, Asuma. Will. Kill. You. It does not help you are playing as Kurenai, a new, but REALLY weak character, Kagura, when fighting Komachi and Towa, is arguably worse.
It is worse because you essentially are playing a Two-Man Squad battle with an already dead partner. Your opponents can regenerate health if they tag out, but you can't.
The last battle against Man Behind the Man Kabuto is especially difficult, because he seems to recover health after being hit much more quickly than ordinary characters do, and you have to finish him off with the Rasengan, although thankfully, you always use the Rasengan when you do a special move against him, regardless of how much health you have.
Seemed to? His health really does recover, even in his playable form, he's the biggest Game Breaker in the game, thankfully, they Nerfed him in the sequel.
And the sequel topped it, apparently the final boss, Deidara, is given super armor and spams his bombs, and since you're playing as a rather Nerfed One-Tailed Naruto, you are going to be throwing your controllers across the room.
Lets not forget the fact that if he uses his bombs right, he can cause extreme damage or even OHKO at FULL HEALTH!
Fighting Chomei, the Seven Tailed Beast during the "Last Battle" from Naruto Shippuden: Ultimate Ninja Storm 3 story mode as Kyuubi Cloak Naruto due to the fact that he's pretty much constantly in the air, meaning you have to jump into the air to be able to attack him. And good luck with that, considering the camera refuses to be remotely helpful. And when you fail, you have to start back after defeating the first two bijuu, which also means you have to sit through the two flashbacks that appear during the section. And after all this is done, you still have to defeat Tobi, who hits hard and loves to spam Kamui to constantly dodge your attacks.
The fight between Son Goku and Chomei, while the former is fast and attacks hard while the latter spams projectiles.
Death is almost always That One Boss in each Castlevania game, and Castlevania Judgment is no exception. Well, except that he's a playable character in that game, so the honor instead goes to the similar-looking Time Reaper. Despite his lack of hitstun, most of his attacks can be easily blocked or evaded, until he busts out an unblockable sweeping Kamehame Hadoken that can take out your entire lifebar with one move if you aren't careful. At harder difficulty levels, he starts using two at once and starting them in different places. At the hardest difficulty, you have to dodge flying homing skulls at the same time. And if you can't lower his three lifebars quickly enough, he'll keep using them. Despite infinite continues, it can be difficult to beat him at the hardest level at all, and the game knows it: you receive a Cosmetic Award for every character that manages the feat.
Despite being a playable character, Dracula also counts, usually only appearing in boss-like matches and being hard as hell - yes, he walks slow as molasses, but he can teleport behind you in a flash, leave traps everywhere, and has insanely powerful projectiles that are even more deadly at close range. To top it all off, he has one of the highest defenses in the game (read: longest life bar), even unmodified. Easier versions will hand your ass to you until you get his tricks figured out. Harder ones will kill you even then.
Metal Sonic is Sonic the Fighters' answer to Dural. Not only does his repertoire consist of attacks used by other characters, but also can (and will) unleash a beam cannon of complete and total annihilation from his stomach, which takes off a large chunk of life.
And in Sonic Battle, we have Emerl. No, not the Emerl that you took control of while playing the game. We're talking final boss Emerl, whose hopped up on the seven Chaos Emeralds, and has access to every single Ultimate ability in the game, from basic movement, to attacks themselves.
Guilty Gear original has Justice. Her Gamma Ray will chip you for massive damage even if you block it, and, in a twist, a mechanic available to all character actually helps Justice even more: when a character is nearly dead, they get infinite supers. Guess what Justice does when she's nearly dead?
Expanding on Justice, she also has a move set that covers a great range of the screen, her Michael Sword covers basically the whole horizontal range of the screen, her Imperial Ray covers all the space in front of her and if its done at point blank your health is screwed. Worse still is that its a special in GG 1 as opposed to an Overdrive, meaning she can abuse it all she wants. If she does it after a throw she can combo into it infinitely, her Valkyrie Arc has a good amount of invulnerability frames and can counter nigh on everything, she can improve the range of the moves by dashing while performing them, and her Gamma Ray overdrive dizzies you full stop should you get hit by it, and you can guess exactly what will happen if her health is low enough that she can fire it off repeatedly. Worse still is she's as fast as Chipp and hits like a lorry.
Guilty Gear X has Dizzy. There has been more than one incident where Boss Dizzy has won two rounds against the player before the Guitar part of her theme tune has begun it's first chord. Even if you don't die, she has Triple Jump and Double Air Dash. The two fastest characters in the game, Chipp and Millia, only have one of the two abilities mentioned above (3x jump and 2x AD respectively). In X2, Dizzy lost her triple jump.
GGX 2 has I-no. Megalomainia is a pain for newbies, though somewhat easy to avoid if you know the pattern. Thanks to the guard bar mechanic, as you block more hits, you take more damage (chipped or otherwise) and stun, while the inverse is true for taking damage. Megalomania hits too many times and will raise the guard bar to full before you know it, so if you do get tagged by the attack, you will take somewhere around the full unblocked damage unless you start holding Faultless Defence. What gets more experienced players though, is her air super where her hat screams and sends shockwaves, interrupting whatever you just tried due to the fast startup and invincibility frames, and because it's Boss I-no, the attack will take off about 80% despite the guard bar stepping in to intervene. Don't forget that I-No just loves to break out of any and all combos you have and instantly use Megalomania.
As one Youtube commentator puts it:
I've never been more angry watching a girl take off her top.
...for the twelfth time.
Also, she has the annoying tendency to set up the warning screen for one of her Megalomania patterns, and then change it before firing. You might think you can be cheap and jump behind her if you're using an agile character, but for some bizarre reason, if you do this, or attack her while she's using Megalomania, it sets off as if the attack hit you and the orbs fill the screen murdering you. Granted, though, once you know how to avoid Megalomania by watching the warning patterns and acting accordingly its easy to hit back.
And the PS2 version of X2 has Dizzy again in story mode, who just about matches her previous self, plus has an Instant Kill super that takes up the entire screen.
Immaterial and Missing Power brings us Yukari Yakumo, who serves as the next-to-last opponent in almost everyone else's scenario. All of her story mode spellcards can be difficult, but her fourth spellcard, "Infinite-Speed Flying Object", is by far the worst. Thick lasers will shoot across the screen at random heights (as many as three at a time), and cause high damage, have top priority, and are completely unblockable at higher difficulties. And they won't stop firing until you deplete her entire life bar, a feat that could be a pain to accomplish when she starts blocking a lot during this spellcard on higher difficulties.
Yukari returns in the second fighting game of the Touhou series, Scarlet Weather Rhapsody, with a whole new set of evil spellcards, including the ridiculous "Doppler Effect" spellcard. Meanwhile, Remilia Scarlet seems to have stolen those unblockable lasers for her "Remilia Stoker" spellcard.
What makes Yukari's Doppler effect really bad is the fact that the projectiles, while few in number, move in a pattern that is impossible to dodge, unless all you do is block and graze non stop, and each wave is timed to hit you at the precise moment you recover from the previous hit, meaning if you get hit once, you're done for. The easiest way to overcome it, is to save up a powerful spell card and use it on her when Doppler effect is activated, but before she sends out the first wave, and pray it takes her out in one shot.
There IS a reliable way to dodge Doppler Effect - graze back and forth on the ground under her and switch directions every dash so you won't ever backdash. Since grazing the orbs does not drain spirit on the ground, you can do it indefinitely... in Remilia's, Reisen's and Iku's story mode, that is. At the end of Youmu's dash, she no longer counts as grazing, so she's still screwed.
Tenshi Hinanai, also from Scarlet Weather Rhapsody, has several incredibly irritating spellcards when you fight her in story mode. However, after you finally win, she pulls out one last spellcard, "Scarlet Weather Rhapsody of All Humankind." At the end of this card, she starts shooting Wave Motion Guns at you, with very little time in between. It looks easy, but it really really isn't. Getting caught in one gives you almost no chance of getting out, letting her Beam Spam you to death while you can't do anything.
Iku Nagae, the other new entry in SWR, has a spellcard that makes her untouchable to about half the cast with only very brief moments of vulnerability.
Kron, the final form of Syn, from the PSX game Cardinal Syn. Gigantic dragon in a fighting game full of human/humanoid characters? Check. Unblockable fire-related attacks? Check. Ability to fly out of the other character's reach? Check. One-hit-kill move (i.e. biting the other character in half) that she loves to attempt to perform quite frequently? Check. Can absorb a lot more damage than any other character? Check. Doesn't even have a timer on her stage, so it isn't possible to get a cheap win by hitting her a few times and running/dodging out of her way? Check.
N. Boss in Clay Fighter. Not only does it continually shoot white balls at you, giving you no time to attack, its design screams of lack of effort. It's two eyes over a ring of clay balls. That's it.
There are certain characters in M.U.G.E.N known as "cheap characters". Light Yagami is a special case, as he kills in one hit, and the opponent needs super speed to be able to kill him. See here.
More specifically, any character that kills you instantly, with no chance of counter. Light Yagami is merciful... you get to move.
The None's Giygas. Getting past the fact that he is possibly one of the creepiest and most frightening bosses one could face, fighting him is like facing a gauntlet of monsters. First off, his first form is nearly impossible to lose to, it's very easy. Then the second form pops up. He has everything from PK Rockin, to Half Life head crabs, to 2 grabs that can instant kill. The worst part? He's floating around the arena somewhere and blends in far too well.
There's several offenders in BlazBlue, but the most notable are Unlimited Rachel, Unlimited Hakumen, Unlimited Nu-13, and Unlimited Ragna. ESPECIALLY the last two.
In the Sequel/Rerelease, BlazBlue: Continuum Shift, they add Unlimited Hazama to the mix. He is exceptionally difficult, with attacks that can only be called unfair.
Unlimited Tager. The original Tager couldn't dash, this one can- well, it's more like a jog, but it's still faster than his walk. Not to mention he's invincible for the duration of it. Also, his Electric gauge starts out full and is split in two, meaning he can use Spark Bolt, his only projectile and a strong one at that, twice at the beginning of a match. His magnetism is also stronger, to the point where he can hit you with a powerful punch that sends you flying to the wall even if you're on the other side of the screen. Additionally, his Distortion Drives hurt like hell.
Jecht from Dissidia Final Fantasy. The rest of Tidus's section is extremely easy, but Jecht will hammer you into a fine paste. He's also much harder than almost all of the other villain fights surrounding him. Part of the thing that makes him so difficult (asides from being a legitimately terrifying opponent) is that he has access to the Magic Pot summon. The Magic Pot summon will copy the opponent's Bravery once in the fight, which indicates that Jecht is either going to have a sufficiently higher bravery just before you break his. The dire threat to this fight is simply in the playstyle of Tidus, which, unless the player practiced sufficiently, is going to run into Jecht Block. Often. Even if the player has practiced with Tidus, it is unlikely at that stage in the game in which any of Tidus's counters have developed into the useful counter-chain Quick Hit, so he's got to use the HP attacks that are Jecht blocked.
Golbez in Shade Impulse as well. The A.I. will give you a very small taste of what a devastating Golbez main can really do. Each of his attacks are potential 1-chain breaks because almost EVERY bravery attack he uses chains into an HP attack, meaning, basically, that you cannot allow him to hit you, ever, if you want to live.
Much moreso would be Chaos, the end game boss. Never flinches, huge Brave and HP, powerful summon, fast, powerful attacks? Par for the course as an end game boss. Stats so high he can Break an equivalent level character with the best equipment available in one combo, attacks that cover the whole (and tiny) arena, most of his attacks nigh unavoidable and one actually IS!? Plus, when you face him at the end of the story he's level 50; he can reach level 110.
His summon deserves further elaboration. When players use a summon, they are limited to once per fight and restricted to that summon's effect: Odin has a 50% chance of inflicting Break, Rubicante triples your bravery, etc etc so on and so forth. Chaos's summon? Chaos's summon can be used however many times he likes, and it has all possible effects—and, when possible, souped-up effects not available to the player (Your Leviathan drains a set amount of bravery over a set period of time, his Leviathan drains more bravery over a longer period of time). Basically, Chaos can freely manipulate his and the player's bravery stat, and as such if he decides that the player is not going to win, then the player is not going to win.
The level 50 one is easy enough if you've powerlevelled your character of choice. Inward Chaos (level 110) is much rougher.
The sequel has Firion vs. Ultimecia. Ultimecia is level 20 and Firion is 8 unless you wish to suffer a penalty. Ultimecia will keep to the air, so Firion must take to the air. The problem with this is that Firion's Air game, while improved from the first game, is not enough against an opponent who spcializes in long range, aerial assaults.
Gundam VS Series has a few with NEXT PLUS. Good luck surviving against the Wadom or any ranged boss with a melee heavy suit like Exia or Epyon. This trope found both of them frustrating to kill as the Wadom's walking is a melee in itself. Think of Exia or Epyon as two well armed swordsmen having to destroy 2 scythed chariots and you will get the Wadom fight.
Extreme Vs has Extreme Gundam Tachyon Phase, one of three possible forms for the Extreme Gundam. Very powerful melee combos, a quick shockwave attack that stuns you, a long-range dash that allows it to cross the battlefield in seconds, it can summon Sword Attack Drones, and it can extend its sword to massive lengths. Extreme's other two forms focus on long-range attacks, which, while just as powerful, are also much easier to dodge.
Also from Extreme Vs: the Rafflesia. Its tentacles are far longer than they should be, its beam attacks have incredible homing properties, nearly all of its attacks stun you for several seconds (thereby guaranteeing that you'll get stun-locked into massive damage), and it can attack in all directions... the Mobile Armor doesn't have a "front" or a "back", all sides are equally dangerous. It can even execute multiple attacks simultaneously, though thankfully it doesn't usually do this unless it's actually fighting multiple opponents at once.
In Tatsunoko vs. Capcom, the final boss Yami could be considered this, his final form the most frustrating, with attacks that can cause an entire life bar to die, and his attacks can be incredibly difficult to dodge.
ESPECIALLY if you're using Gold Lightan, as Yami's supers, especially the one where he grabs you, are literally impossible to dodge without EXACT timing, the only way to get out of it is to Mega Crash, and even then doing THAT hurts you.
Yu Yu Hakusho: Dark Tournament had Young Toguro as a problem in the final round. While most of the game had you fighting enemies in less than optimal conditions such as lack of health or spirit energy but within reasonable limits against beatable opponents on a rising difficulty curve, Young Toguro suddenly steps up the difficulty a hundred levels spiking way beyond anything you've fought so far. He must be beaten in a time limit, one combo from him will end your health bar (and that can be literal on many occasions if he hits you with all the blows), and he has a second health bar hiding off screen. So it's essentially facing him in two rounds without healing. Not to mention the worst part is unlike everyone with realistic blows, the hit box for Young Toguro's attacks are almost a foot out from where he attacks. If you guard, he will throw you. If you attack, he will block and hit you during your recovery. If you use spirit energy, he will either take it head on and retaliate against your drained ass, or he'll block it and hit you with one of his own. it also doesn't help the combos for the game are ridiculous, so mastering the game is required. The worst part: after beating him you still have to defeat him again in "Dark Tournament Plus" mode except the difficulty is slightly ramped up at the beginning which means by the time you get to him... forget it.
The final fight with Genkishi at the Melone Base in Flame Rumble X and XX. His normal sword attack can take out a good third of your health and its animation in near instantaneous. His favorite trick however is to go into the background and drop skeletons on you. This is not helped by the fact that he does not recoil from damage and can leave in the middle of your attacks so if you wanna say hit him with your counter move chances are he'll knock you out of it or just go into the background. It also doesn't help that collision detection in this fight is more or less a complete crapshoot leaving you to either phase right past his body or crash straight into him when you want to dodge. The cherry on top has to be his lines which all border on Most Annoying Sound territory except for his laugh (which is squarely in that territory) which is played when he goes into the background.
The Final Boss, Master, easily counts for this despite the fact that she's the last boss considering that most of her attacks revolve around using BFS's to completely destroy your character's health. Her basic attacks involved using a huge sword to slice your character multiple times for an equally huge chunk of your health, and throwing several huge swords across the field for huge portions of health, which makes closing in on her or fighting her at a distance even tougher to deal with than Uranus, which shows why she's only the second to last character you fight. After a while, Master'll start to use her super attack where she dashes with a huge sword to take out most of your health if it connects, and if you were lucky enough to take her down to low health, she could use her Desperation Attack where she spams an insane amount of projectile swords which basically counted as a One-Hit KO if it connected. This was difficult for characters who could fight at a range, but an absolute nightmare for those who couldn't. The craziest thing about this was the fact that there were unlockable hidden characters in the game who were complete Game Breakers, and they would still struggle to beat Master.
Even oldschool fighters weren't immune to this. Mutant Fighter (a.k.a. Death Brade) had the Doppelganger, which changed into whoever you were playing as. Just an ordinary mirror match, no biggie, right? Except that after a little while, it would look EXACTLY like whoever you were playing as, making it really hard to figure out which one you're controlling. Plus it had nearly airtight striking defense. And it would win at least 80% of its grapples no matter how quickly you mashed the buttons. On top of that, it could take lots of punishment. Lots...and lots...and lots of punishment. Nearly everyone took longer to beat it than the Demon or Archmage.
The dual Black Battlers from Ougon Musou Kyoku. Not only does he start with 5 SP, but he begins with everyone's special effect. If he gets his meta special in, you'll be juggled in the corner until you lose.
The Eternal Champion from, well, Eternal Champions. He can use any of his moves regardless of his energy meter, does a ton of damage with every hit, can actually make himself invincible at any point in the match for a few seconds, and requires a lot of hits to take down. Oh, and there are FIVE FORMS, all of them with their own moves. And there's no continues once you reach him; lose, and it's game over. Yikes.
He's even worse in the sequel. While he doesn't cheat quite as blatantly, he's still ridiculously overpowered. And this time, he comes with NINE forms. And then you have to fight the Dark Champion, who not only has the same number of forms, but is even more powerful than the Eternal Champion. Oh, and unless you're on the lowest difficulty, there's still no continuing. So, that's a total of eighteen bosses you have to beat on one credit. If a fighting game is allowed to have a Marathon Boss, these two would be some of the clearest examples ever.
First, you fight against Galactus' heralds at the same time, while you can only have one character on at a time, and they have infinite-use supers. However, they share a lifebar.
After they go down, you fight Galactus himself, who has extremely high health and defence, and does a ton of damage with his screen-filling attacks. When the fight starts he'll go easy on you with mildly telegraphed but deadly strong physical attacks, one of which is a finger flick that counts as a Snap Back, wherein your current fighter is booted off the field and another one forced into play, a grab that must be super jumped or button mashed to avoid, and a vicious Megaton Punch. Once he's down to half health, depending on if you wailed on him hard enough, he'll stagger, signifying that he's not pulling his punches and starts using his Power Cosmic on you consisting of various beams. Hilariously his eye laser can be avoided by getting right in front of him, but his other lasers aren't so easy, two of which (A Beam Spam from each finger and an array of beams that cover the screen) have to be blocked if you're not using a character who can fly, and they still cause massive chip damage, god help you if you're using Zero, Akuma or Phoenix who have the lowest stamina in the game. Once you've hit him hard enough he's likely to use a Sphere of Destruction that can instantly kill you if you don't deal enough damage to stop him. Plus, if you lose against him, he destroys Earth (EARTH K.O.).
To make his SNK Boss status even more apparent, the Ultimate re-release allows you to unlock a mode where you can play as him. (And before you ask, it's single-player only, obviously) Put simply, even a novice player has to explicitly try to lose in this mode, which is easier said than done, as he wins if the timer hits 0.
In Don King Presents Prizefighter, Chad Dawson is the 10th to last opponent and worth less money than each of the nine you face after him. He is by far the most difficult opponent in the game. He moves around a lot, has tremendous reach, hits like a bus, and recovers stamina three times as fast as normal. On Champion level, it will almost certainly take you at least three attempts to take him down. Terry Parkhurst, the world champion, is an absolute creampuff compared to him.
The final boss of the first Clay Fighter game, known as N. Boss. He knows all projectile based attacks in the game & throws them constantly at his opponents. He also has a sick grab move.
Superman and Regime Superman in the iOS version of Injustice: Gods Among Us take a hell ton of hits to even defeat and have terribly overpowered heat visions to one-hit any of your characters at their disposal.
In the Battle Tendency part of Story Mode in Jojos Bizarre Adventure All Star Battle, you replicate the final battle of that arc: Joseph Joestar versus Kars, the Ultimate Life Form. As you would expect, Kars spends the entire fight as the Ultimate Life Form (a state he can usually only achieve with his strongest Super Move); however, he also has regenerating health and Super Meter. Bear in mind that, in the manga, Joseph won this fight by running away and pulling off a trick involving a volcano. You have to win in hand-to-hand combat (but running away is actually one of the Secret Missions). Expect to try this one over a few times.