That One Boss/Action Game/Megaman



Mega Man stands as a shining example of classic Nintendo Hard. These guys are the reason why. It doesn't matter if you've got the select trick or a folder full of Program advances, these mechanized murder Masters are coming for you.

Please keep in mind that if there's an easy method to take out a boss, they're not this trope. Remember as well that We Are Not Game FAQs, your example shouldn't read, "Unless you do..." or "But if you have..." That One Boss is the guy who kicks your ass even with a trick. This being Mega Man, every boss has a weapon that they're weak to. If that weapon turns out to be impossibly frustrating to use, that boss probably belongs here.

Bonus Boss is now banned from being That One Boss. It's optional and you know it's overpowered, nothing prevents you from not fighting it.

Mega Man (Classic series)

 * It should be noted that the Mega Man Anniversay Collection features an easy mode that covers the first seven games. The NES games benefit with Mega Man's defensive and specials weapon power being upped by 50%, buster shots do an extra unit of damage, while 7 doubles the power of every weapon. The bosses can still be a pain in the ass though.

Mega Man 1

 * Elec Man is possibly the hardest boss to kill with only your pea shooter in any game. He runs around the arena like he's on crack, throws nearly impossible to dodge Elec Beams at you and can kill you in three hits! Granted he's very weak to the Rolling Cutter (Cutman's weapon), but if you didn't quite figure out that bosses have weaknesses... There are ways to make fighting him easier that involve some rather ingenious AI exploits, but even then they're still easy to screw up and require a cool head and precise timing that you can't deviate from lest he continue his spastic rampage.
 * Fire Man, mainly because his Fire Storm attack is very powerful and you have to have perfect timing to jump over it. Also he spams this move like there's no tomorrow, so even having the Ice Slasher isn't going to guarantee you victory.
 * Ice Man seems simple on paper: He moves around only a little bit while jumping and firing three Ice Slashers in a very easy to memorize pattern that he never deviates from. However, his Ice Slashers are rather large and a bit unwieldly to jump over, and they take away one third of your health should you get hit by one. While Ice Man should avoid this status in theory, the fact he's surprisingly durable and that three mistakes will kill you can make him deceptively tough to beat.
 * The Yellow Devil from Wily Stage 1. An odd change of pace from the game's Robot Masters, Yellow Devil is an enormous blob of yellow goo that is invulnerable for a good deal of the fight thanks to the fact that he's constantly having sections of himself rocket across the boss room at ludicrous speed in hard to dodge patterns. When he forms, you have only a few seconds to shoot his exposed eye before he goes back to his separate and reform attack, and to make things even worse, his eye can spawn in a position that's too high to hit! Many gamers had lost a good chunk of their childhood trying to get past this shining example of Nintendo Hard. Unless they cheated.
 * The worst part about the Gamecube version Anniversary Collection is that the glitch that makes the Yellow Devil easy to beat was drastically changed (the timing was changed to make it less effective if you knew the change, worthless if you didn't), thereby making the fight that much more difficult.
 * Then the Rainbow Devil shows up in Mega Man Zero. It's just as cheap as previous models. It has an especially annoying attack where it bounces around the room, splits into two parts, then into four before finally reforming. It also has a punch attack that crosses nearly half the arena and a projectile attack that makes nearly the entire ground a danger zone. Good times. Thankfully, the redux in Zero 2 is actually somewhat easier.
 * And there was also Dark Moon, the Devil-type boss in Mega Man V for the Gameboy. He's hell to beat with his own weakness, the Photon Missile, and his separation attacks, the top two rows would bounce down in order to try to kill you.

Mega Man 2

 * Air Man.
 * Strangely, this is a reversal of an earlier viewpoint - Air Man used to be considered almost as weak as Bubble Man, and a good boss to start the game on. This may in part be due to the American manual having a walkthrough for his stage.
 * Note that the Japanese original of the game (which spawned this particular bit of Memetic Mutation) only has Hard mode. This means you do half damage to all bosses, and some enemies too. The US version was made easier be default, and Air Man is affected by this. On the US version's Normal difficulty you can damage him much faster than he can damage you, but you actually have to learn to play on the Japanese version (Hard mode in the US). Furthermore, no matter what version you play, one of his tornado patterns cannot be dodged.
 * Quick Man, if not because his stage is That One Level it's because he is incredibly fast (hence the name) and has a completely unpredictable attack pattern that makes fighting him more akin to fighting a spastic chimpanzee, and does almost 1/3 of your life bar with Collision Damage! Oh, and there is an attack that can take half his life in one shot, but odds are you will probably need it to get past the almost impossible laser barrage right before his door! And even then, you are only taking half of his life energy, and he's still more than capable of killing you from that amount of health. And to make matters worse? Jumping right into him is easy to do since his boss room consists of raised floors that need to be jumped over to traverse.
 * You'd think that a robot that weaponizes leaves would be pathetic, right? WRONG. Meet Wood Man, whose main method of attacking is shooting a couple of large leaves into the sky and having them descend in hard to dodge patterns. While this is happening, Wood Man generates a Leaf Shield that blocks all methods of attacking before tossing it at you in a somewhat tricky to dodge attack. And he takes a leap at you with every throw of his Leaf Shield, meaning that sooner or later, you're going to have to run underneath him which is risky thanks to his large size and the lack of a sliding mechanic. Thankfully he's a breeze with the Metal Blades or especially the Atomic Fire, but otherwise save this guy for later.
 * The Boobeam Trap, The turrets with blindingly fast shots from the walls heading straight for you every three seconds or so. It's the other boss in Wily's Fortress that only has one way to damage it. And the kicker? You only have seven uses of the weapon maximum. Five to actually destroy them, and at least one to break barriers blocking the others. If you don't make clever use of Items and/or accidentally miss a shot or two then you will not be able to defeat this boss. And since you don't get energy refills after death you either need to take forever restocking energy off the few enemies or just game over entirely and restart the stage. There's a reason why practically every MM2 romhack out there, even the obscenely hard ones, generally make this boss easier.
 * The second phase of Wily Machine 2 is a real big step up from the pathetic first phase. Basically, the machine fires projectiles that constantly bounce off the ground that can be awkward to dodge at times. This shouldn't be overly hard in theory, but the bouncing shots have incredibly deceptive hitboxes that allow the blasts to hit you even if you aren't even touching them, and they do quite a bit of damage. And whenever he fires them in volleys, you pretty much will be hit because it's literally impossible to dodge them. Even when you hit him with his weakness (Crash Bombs), the battle's outcome is pretty much down to you hopefully doing damage to Wily faster than he does it to you. Sure, he's no Wily Machine 7, but he's still a real hair-puller either way.

Mega Man 3

 * Shadow Man. He always hops a few times, but there's no way to predict if he'll follow up by sliding into you or tossing a few Shadow Blades. Both are not only quick and hard to dodge, but will hit rather hard as well. On top of this, the only weapon that can hit Shadow Man effectively, the Top Spin, has to be used at very close range, and its weapon reserve energy drops like a stone when you are using it while in direct contact with Shadow Man. What tends to happen is that if you try attacking directly with Top Spin, ALL of its weapon power can get drained when you land your first hit. Then Shadow Man still has 75% health and you have no Top Spin power left. Good luck hitting him with the Arm Cannon, and even more so, good luck in having him use a manageable pattern.
 * Needle Man is just as bad, thanks to being a complete and utter spaz thanks to his unpredictable AI. He's deceptively fast for his bulky build, can take a ton of hits before going down, and fires his needles at machinegun speed. Even worse, the Gemini Laser (his weakness) is slow to and easy for Needle Man to dodge, meaning that you're better off just going after him with your buster. Good luck if you're trying to do a no-damage run against him.
 * The Doc Robots take the abilities of ALL the Robot Masters from 2 (You fight two per level, and they make you play through the four harder stages revamped to be harder), but make them much harder to fight thanks to various different factors. If you thought dodging Wood Man's swirling leaves was bad before, try it when they are even larger! Try fighting Crash Man without an attack that can shoot up! TRY FIGHTING A LARGER, FASTER QUICK MAN!
 * Oh, and since it's Mega Man 3, you now have to try and figure out which weapons the Mega Man 2 robots are weak against, all over again. Almost none of them are intuitive either. Further bonus because, even when you do figure out their weakness, they aren't as effective as their weakness was in Mega Man 2 (nothing can break through the Leaf Shield, for example).
 * Bonus points for Doc Wood Man, as the Leaf Shield has both larger leaves and larger shield radius -- large enough that you almost need Rush Coil to get over it (which consequently smacks you into the leaves coming from above).
 * Remember how easy Flash Man was in Mega Man 2? Well, Doc Flash Man is here to remedy that! Thanks to the fact that he's a Doc Robot, he's bulky and tall which makes it harder to jump over him, and uses the Time Stopper much more frequently. His spread shot covers more ground and hits harder, and lets not forget that Doc Flash Man lacks his original counterpart's fragility!
 * As mentioned above, Doc Quick Man for sure. Quick Man was fragile and not too hard hitting, yet still is infamous for being one of the toughest bosses in Mega Man history. Now make Quick Man bigger (and thus harder to dodge), more powerful, and durable while keeping his spastic pattern intact. Uh oh.
 * The Holographic Mega Mans (Yes, that IS their name) in Wily Stage 3 can be easily killed at the start of the stage if you know what you're doing. If you don't however, then good luck finishing the job. While there are three Mega Man clones running around, only one can be hurt and he'll always changes position whenever he and his brethren disappear and reappear. Their vanishing act follows no real pattern, and there's absolutely no way to tell where the damageable clone went outside of you braving their insanely hard to dodge buster shot barrages. Slip up, and you'll be slaughtered remorselessly.

Mega Man 4

 * Bright Man, who's basically a much tougher version of Flash Man from Mega Man 2. Flash Man just ran around and occasionally froze you in place so he could hit you, but Bright Man does the same thing with a much higher AI and is much more aggressive, prone to firing hard to dodge buster shots while leaping at you and dealing surprisingly strong collision damage. Even worse, there's almost no way to tell when he's about to freeze time, and even if you jump to avoid his oncoming attack, he WILL take a flying leap and nail you anyway. May god have mercy on the soul of any poor fool trying to do a perfect run against him.
 * The whale minibosses called Moby in Dive Man's stage can be absolutely irritating to battle thanks to their fast and hard to dodge homing missiles. While their mine attack seems much more manageable, you have to do so while you're being sucked towards them, which also happens to be right towards a pit of one hit kill spikes.
 * The first battle with Wily is absolutely brutal if you don't realize you can . He fires a constant barrage of huge fireballs bigger than Mega Man with no discernable pattern. And yes, the weak spot on his machine is just a bit too high to reach with a straight shot at your apex. Pharaoh Shot which can be aimed upwards, doesn't work. If you don't know how to , you're going to have to self-terminate on that level over and over until you've stockpiled all 9 energy tanks if you want to win.

Mega Man 5

 * For the most part, Mega Man 5 has tons of easy bosses. However, there's still one that is quite tricky, and that's Charge Man. Mainly, Charge Man's difficulty stems from his erratic movements which make it very easy for him to run right into you while you're attempting to jump over him, and his collision damage hurts quite a bit. Then, there's the fact that he's very prone to going berserk by turning red and launching coal into the air, then usually following up by ramming you. He can't be hurt at all during this state, so it's very easy to waste a charged shot on him while also being nailed by all these attacks he's throwing at you at once.
 * The third Dark Man boss is easily the toughest of the Dark Man quartet. His basic attack where he shoots five projectiles that are tough to dodge is bad enough, but then there's the attack where he shoots three ring projectiles at Mega Man. They can be hard to dodge too (especially in close quarters) and if you get hit, you're doomed to be frozen until Dark Man hits you, which he WILL do, and it hurts a lot.
 * Big Pets from Wily Stage 1 first starts as an irritating Puzzle Boss: thanks to the fact that his platform body segments only shoot out if you him, it can be hard for people to predict when he's going to fire them while attacking the smaller enemies he summons. Even when you pick up on the method of hurting him, you still have to get a clean shot on Big Pet's head while small, annoying enemies constantly bombard you, making it harder to do than it should. And keep in mind: if you get hurt while going for a charge shot, you lose the charged effect, and those little pests will do their best to throw you off your game!

Mega Man 6

 * The Power Piston in Mr. X Stage 2, DEAR GOD. This monstrosity is attached to the room's right wall and constantly zooms up and down juuuuust out of range for your buster while it barrages you with bullets. If that wasn't enough, the stupid thing is constantly raining boulders on you from the sky in unpredictable patterns as well, and they do splash damage when they shatter as well! The boulders combined with the bullets make this stupid thing bullet hell, and thanks to its erratic movements and tiny hitbox, it's a complete and utter chore to hit this thing. God have mercy on your soul if you run out of Silver Tomahawk, because the next best thing are charged Mega Buster shots, and if you get hit (which you probably will) you lose the charge!

Mega Man 7

 * Slash Man, who's fast, strong, and can immobilize you with red goo that falls down at random, leaving you with little time to dodge. Even when you bring his weakness, he still spends most of the fight out of your range, so he's still quite likely to rough you up bad.
 * Freeze Man can be tricky thanks to the fact that his icicle attacks can be tricky to read, and his main method of attacking is to leave you a sitting duck by freezing you in place. His rather quick moving speed and somewhat wonky AI don't help matters at all.
 * Guts Man G of Wily Stage 1 is basically Guts Man on steroids. His Super Arm attack now drops a humongous bolder that will likely crush you, then he smacks it into the wall giving you a tiny window for error when it comes to dodging it. He can also charge you with an almost unblockable dash attack and will toss you into the ceiling for extra damage. Add in the fact that he's ludicrously powerful, is sturdy even against his weakness, and a small weakpoint that only takes damage when it feels like doing so, and you've got trouble.
 * The floating oni head HannyaNED of Wily Stage 3. The screen autoscrolls, it has a plethora of hard-to-avoid attacks such as barrages of bombs and Eye Beams, and its weak point is very small and can only be reached by jumping on the missiles that only stay on screen for a fraction of a second.
 * The final boss also fits here. He is definitely overly hard by the standards of final bosses for the Mega Man series. His weakness, the Wild Coil, is (as per the tradition) difficult to hit him with. His main attack is shooting four elementally charged orbs in your direction, then they stop in mid-flight to adjust their path. And they hurt. Fortunately, he's at least damaged by some other, easier to hit with weapons, such as Freeze Cracker. Word of God says that even the developers found this one impossible without E-Tank usage. Even has some Lampshade Hanging, in which Mega Man attempts to kill Dr. Wily following the battle.
 * This battle is so bad that the creators actually used it again as the final battle in Mega Man 8 and significantly lowered the difficulty. When a final boss battle is so tough, that the programmers actually redo it in the next game just to apologize to fans that struggled getting through it initially, it definitely qualifies for That One Boss.

Mega Man 8

 * Astro Man, if you don't know the way to dodge his Astro Crush, he will destroy you easily and quickly. Oh yes, and he's also the boss of That One Level, so have fun with that.
 * There's the first Wily Fortress boss, that can only be hit by the Mega-Ball, a weapon that has to bounce off of the wall up to the ceiling at the right angle to hit the boss. It doesn't help that its level is pretty tough to get through normally.

Mega Man 9

 * Tornado Man, mainly because his attack is very difficult to dodge and his weakness weapon is a ground based attack and he spends most of the time flying in the air.
 * What's worse than one Devil? Two of them at the same time. Both the yellow and green devils sent blocks of each other right at you at the same time. The pattern is actually easier this time, however, but the core flying out to switch blobs in the middle of it all makes it a little annoying. Inti Creates, you clever bastards...Fortunately, the pause-glitch works on it.
 * The elephants from Concrete Man's stage. Making it worse is that they're only minibosses. And a later Wily stage brings them back.
 * Concrete Man himself is a pain; he hits very hard, his weakness, Laser Trident, doesn't do much damage to him, and he has an unavoidable charge attack if you're trapped in concrete. While the Trident can break the blocks, it doesn't work if you're stuck in it--and they travel diagonally from the air and are very fast.

Mega Man 10

 * There was one character who both inverted this and played this straight depending on the circumstances. I'm talking about Strike Man, who is the easiest robot master to beat with his weakness weapon, but the hardest to beat if you don't. He's weak to one of the most powerful weapons in the game, so beating him with it is a snap, but the reverse is also true, as without said weapon this guy's a nightmare to face. His weapon can bounce off the wall and he can have multiples of them at once. It doesn't help that he is also very fast, very strong, and the boss of a level who has not one, but two annoying midboss fights. I hope you have his weakness weapon, or a lot of E-Tanks, because if you don't, well good luck dealing with him.
 * If you're using Proto Man, you can at least slide under him when he bounces toward you, unlike Mega Man. This is also a double-edged sword however since Proto Man has half the defense of Mega Man, and therefore takes twice the amount of damage if he gets hit, so hopefully you're good at dodging.
 * Blade Man, mainly because he's like a powered up Shadow Man from Mega Man 3 and even faster and stronger with multiple projectile attacks that are tricky to dodge. He can also negate his weakness weapon the Commando Bomb if he swings and hits it before it can detonate right next to him, and the bomb itself does minimal damage, meaning the shockwaves from the explosion have to hit Blade Man before he jumps to the ceiling or another wall.
 * Pump Man, mainly because he controls the water current on the floor to work against you and make you even slower than you already are which make dodging his attacks almost impossible, especially as Mega Man who can't slide in this version. It doesn't help that he can negate his weakness weapon the Thunder Wool if he times his attack correctly.
 * Solar Man is pretty much impossible without his weakness weapon, the Water Shield mainly because he can absorb the shots from your Mega Buster and shoot a huge burst of energy right back at you when he has enough power. Good luck if you take him on first, especially since his minibosses are just as tough.
 * The Block Devil. Imagine, if you will, an unholy union between the Pico Pico Master (2nd fortress boss from 2) and the Devils' penchant on splitting and reuniting in definite patterns, throwing in a weakpoint that stays on screen only 1/3 of the battle and a bottomless pit that becomes a Kaizo Trap if you happen to defeat the boss standing on/over a purple block. Oh, and his weakness involves one of the most awkward weapons in the series!

Mega Man and Bass

 * This game is notorious for Dynamo Man, who likes to hop into a recharging rig in mid-battle. By the time you break it, he'll have regained at least a third of his HP (and he'll happily refill completely if you take long enough).
 * Just as bad as Dynamo Man is Burner Man, who resides at the end of That One Level, and has several attacks that deal a lot of damage as well as one that holds you in place as he has his way with you. His Wave Burner is also capable of deflecting shots, and he likes to back you into a corner with it. Fighting him with his weakness, Cold Man's Ice Wall, he becomes somewhat of a Puzzle Boss, having to use the Wall to push him into the spikes on either side of his arena. Thing is, there are only certain frames that he's vulnerable to this, any other time and he'll bust right through it, and you've wasted your ammo. If you mess up too many times, it's easy to run out of ammo, and be forced to go through his agonizing stage all over again.
 * The second boss fight of the second fortress stage. It's above a bottomless pit, you have to navigate randomly appearing platforms that the boss can destroy, one of his attacks will blind the screen, and is just a pain overall. You will fall into a bottomless pit and die multiple times even if you utilize a few weapons based tricks to stay in the air and/or keep damaging it.

Mega Man Powered Up

 * CWU-01P, especially when playing as Rock/Mega. His main way of hurting you is Collision Damage. Sounds easy, right? WRONG! At least on the aptly-named Hard Mode, he's damned fast. And as the aforementioned Joke Character, you can't hurt him without getting right in his face and kicking him enough to first get rid of his bubble shield, then once more to actually cause damage. Oh, and just for good measure, he does also have another attack -- a frickin' laser beam!
 * Also, if you're playing as a Robot Master, when you have to fight the Master who's weapon is strong against you, good luck.
 * Special mention must go to fighting Cut Man as Bomb Man, as all of his attacks can destroy your bombs. In other words, the only way to do any damage to him is to throw a bomb behind him and hope that he doesn't jump away before the bomb explodes. Lather, rinse and repeat 15 more times.
 * Anything with Oil Man.

Mega Man X

 * The Bosspider, due to the speed she moves at, the difficulty of quickly figuring out where the boss will end up depending on what pole she appears on and where the pegs she moves along appear and the timing needed to hit before she regains its invincibility. And then she Turns Red.
 * Storm Eagle is hard to hit in between attacks because he is about half way up the screen. When you fight him, there is a doom pit on either side. He has a number of hard to deal with attacks, but most of the time just flies around randomly, crashing into you.
 * Try fighting him without getting the Dash Boots first, hope you have some good aim when he's dive-bombing the screen.
 * Armored Armadillo has 3 strategies: guarding, bouncing off the walls and shooting his energy shots. He's invincible when ever he isn't shooting at you, as he will most likely block your attack. Guess what he decides to never do 90% of the time.
 * Now try fighting him on Maverick Hunter X's Hard Mode, where he picks up an attack where he bounces off the walls and fires off bullets in four directions. Even if you use the Electric Spark to take out his armor, it's still hellish.
 * Heck, try fighting Armored Armadillo WITHOUT Electric Spark. Even a fully charged X-Buster shot won't even scratch him.
 * Launch Octopus. He dishes out homing torpedoes like candy and More Dakka is his trademark. When he's not firing torpedoes at you, he's jumping clear across the arena right on top of you. And to make matters worse, he also has a nasty attack called e-drain where he creates a whirlpool that pulls X into his grasp. Oh, and guess what it does when he grabs you.
 * Boomer Kuwanger is the one boss that is almost impossible to beat without his weakness weapon if you don't know what you're doing. You can't just get lucky and pull off a victory with him considering that he constantly teleports and tries to grab you, then slams you into the ceiling For Massive Damage

Mega Man X2

 * If you collect all of Zero's parts from the X-Hunters, then you'll be able to skip the battle against Zero at the end of the game. If you don't, you're in trouble at the end of the game before you battle Sigma. The only weapon that really hurts him- the Speed Burner- is hard to hit him with, and his attacks are hard to dodge. And after you've beaten him (probably draining all your Sub-Tanks in the process), you still have to fight Sigma. The moral of the story is to get all of his parts, no matter how hard they are to get early in the game.
 * Violen is the worst of the three fights you have to do to get the Zero Parts, considering he liked the idea of Armadillo's invincible attacks, but decided to make them even less predictable.

Mega Man X3

 * Crush Crawfish, mainly because if he grabs you with his huge pincers and lives up to his name, he does so much damage it's not even funny.
 * Byte isn't all that difficult if you know what you're doing, but he uses an absolutely cheap and never-before-seen gimmick to utterly crush the unprepared. He throws a device on the wall that slowly repels X. He'll charge at you shortly after it attaches. If he hits you, he'll punch you into the ceiling, wait for you to fall, punch you into the opposite wall, and repeat the pattern. You cannot break free from the punch combo, and it's his one and only attack, so he can easily loop it until half your health bar is gone. He's too big to jump over, so you have to either execute a well-timed up air dash or dash-jump at the wall and use the split-second of hang time you have to dash-jump over him. Oh, and whenever he hits the wall, if you aren't in mid-air, you're on your butt, so you better have hopped over him before he hits.
 * Sigma and Kaiser Sigma. Sigma's swapped out his Z-Saber and Wolverine Claws for a Captain America (comics)-esque shield, which defends him from all attacks as long as it's bared. His main attack consists of spraying fireballs all over the place, which have an unpredictable pattern and do huge damage. They can be avoided by air-dashing to the wall Sigma's adjacent to, so of course he begins throwing his shield to knock you out of the sky. And when you do bring him down, you get to face Kaiser Sigma, who takes up a quarter of the screen, fills another quarter of it with floating mines, has a weak spot about the size of a pinhead, and keeps his back to you for half the match. Oh, and when he's dead, you have to do an escape segment with rising lava eager to nip away your last few bars of health while his virus form harrasses you. He may be the final boss, but he's considered to be easily the worst of all the SNES games.

Mega Man X4

 * Magma Dragoon considering he has both the fire based Hadoken (Fireball) and Shuryuken (Dragon Punch) and loves to constantly spam these moves. This isn't even the best part as when he gets hurt enough he'll shoot out a large stream of fire, or summon a fiery meteor storm that is pretty much impossible to dodge. This is take Up to Eleven with Zero who has to get up close and personal, unlike X who can easily fight him from a decent distance away.
 * Sigma's an exhausting grudge match, with three stages, the last consisting of two separate bosses, and no breaks in between. The first form goes down easily enough, but the second delights in chasing you around with his Sinister Scythe and filling the screen with projectiles, and the third? Each form stays on screen for maybe thirty seconds at a time, and alternates with three robot heads who try to knock off bits of your health and never stay dead for good. One of the two forms is prone to busting out a nasty laser attack with maybe a second's window for dodging, and the second has a nearly unavoidable garbage-spray and an attack that can knock you into some insta-kill spikes. And while X can tear through the first two forms with ease and at least keep the last at a safe distance, Zero has to deal with these things up close.

Mega Man X5

 * Duff Mc Whalen, who's one of the biggest mavericks ever is basically like a whale version of Burst Man from Mega Man 7, has a very difficult attack pattern to dodge, and when he Turns Red, he actually tries to push you into spikes for an instant death which means you have to fight against him blowing you back and hitting him without taking too much damage or getting hit by the spikes.
 * If you thought Storm Eagle was bad, The Skiver/Spiral Pegacion is a nightmare, boasting the same sort of attacks, but all vastly more difficult, coming from all directions at high speeds. Oh, and making him a standout from every other boss in the entire series, he does not have an actual weakness to a specific power. Instead, he can be frozen with the timestop power, which doesn't last long enough to punch off a decent portion of his health.
 * Black Devil in the first Fortress stage is hard for X, but a NIGHTMARE for Zero. Besides a very hard-to-dodge spam attack, his weak point is his eye, which fires yellow orbs at you, and the most effective attack against him is lightning. X has his buster and Tri-Thunder weapon, both of which are ranged, but Zero has to use his saber and the E-Blade, which he slashes upwards while moving forward slightly. This means he has to get practically in the boss's face to use it, but far enough away not to touch the body (which deals massive damage). Add to the fact that sometimes the eye appears in a place which you plain can't reach with the saber, and you get a very difficult boss for Zero to counter.

Mega Man X6

 * Infinity Mijinion, whose main gimmick is cloning himself. His clones have the uncanny ability to get in your way a whole bunch until you forget which one's the real one. It doesn't help that even getting to him is a huge problem because the entire stage is spent fighting a giant miniboss, which is no walk in the park either. Heck, this is a case in which using the boss' weakness probably makes the battle harder. Trying to even hit Mijinion with the uncharged Guard Shell is difficult, if only because it only works against one of his attacks (which is barely used, mind you), but if it does hit, it creates another clone. It wouldn't even be that bad if the clones didn't have so much health, usually taking three charged normal shots to destroy.
 * This is taken Up to Eleven with Zero if Infinity Mijinion does his ultimate move and hovers in the air in the middle of the room dropping an endless amount of clones. X can at least stay on the wall and shoot him, but Zero has to do a huge wall jump from the top of the wall and strike him in midair.
 * DESU BORU!! Er... High Max. AKA 'Assclown'. "Hmm... What's this glowing crystal-like thing here? Oh, wow! Haven't been here before. Guess I can fight the main boss later. Huh? A door? Must be the main boss. Wait a minute... Pfft! This guy's no problem... Oh, wait. I don't have any weapons on me, and I need them. Like, at all. Time to kill myself!"
 * Plus, if you take him on with Zero, his only weakness is a very short range attack, and then you still need to slash him to do damage. Be sure not to touch him though! Then, he Turns Red and starts firing off projectiles four at a time, which, if you weren't absolutely careful before, will probably kill you very quickly.
 * The Sub Boss of Blaze Heatnix's stage. Which you fight 5 times over the course of the entire stage, with exactly 1 segment that doesn't involve fighting it or the actual boss. The worst offender is the part where you fight it while forced-scrolling upwards on very small footholds. Which the sub-boss is bigger than. It doesn't help that there's insta-kill lava covering half the fucking screen.
 * Heatnix's miniboss is cake if you've used X to beat Metal Shark Player or used Zero to beat Infinity Mijinion first. Slap it with X's fully charged Metal Anchor or Zero's Giga Attack twice and it's gone. The real bastard is Gate. Invulnerable to all your attacks, so you have to turn his back on him, and tends to fly right into you. His attacks are ALSO hard to turn back on them, since they need to be level with him, and then hit him in the face as they break from damage. The most damning part of the fight is the arena. There is no floor, so you're liable to fall to your death more often than not. The only good thing about this battle is that you freeze in place when Gate runs out of health, so there's no risk of falling to your death after beating him senseless.
 * The Nightmare Mother(FUCKER!!)... Good God. On Normal difficulty, this boss fits her name to a damn tee. On Xtreme, she cranks this up to insane heights. Both cubes move at an accelerated speed even at full health and can change direction whenever they like, all while constantly firing projectiles at you until they stop. And if they do stop, you better hope the gods are smiling on you, since her attacks can be used by both eyes and combine sickeningly. Each eye can release shockwave-producing projectiles, raining fireballs, floating orbs that shoot lightning or even set the floor on fire. Two out of four of these attacks reduce your dodging room severely, and if they ever set the floor on fire AND shoot the lightning orbs, then you better. Fucking. Pray.

Mega Man X7

 * Snipe Anteator. On paper, his weakness was the "Moving Wheel." In the game, you could barely get said weapon to hit him. Even if you could, it actually did LESS damage than a fully charged X-Buster. Translation: Just use X already. Oh wait, you have to unlock him first! On top of that, he doesn't have a hitstun animation, meaning that he can counter your attacks as they hit him.
 * Then there is Flame Hyenard. You had to damage a mech travelling over hot lava so you could get to him. This is easier said than done since the camera can screw you over. Then you fight Flame Hyenard. He splits himself into clones that shout "BURN! BURN TO THE GROUND!" The weapon he's weak against actually works, but if you're using X or Axl, you'll run out of ammo long before he is dead. And he has a Scrappy Level to boot!
 * And then there is Red. If you don't have Zero, good luck to you. You have to target Red from a distance. Since the camera enjoys screwing you, this becomes a problem. Dodging his attacks is problematic because you're not fighting on solid ground. You're fighting on platforms with a hole in between each one. Fall and you die. Get hit while jumping and you will die. He constantly teleports around on those platforms and there is no way to predict where he will end up. The only way to finish the fight quickly is to use Zero and take Collision damage while slashing him.

Mega Man X8

 * Bamboo Pandamonium. He's big enough that you can't get behind him without taking a hit, and has attacks that take up big chunks of the screen. Also, his desperation attack hits like a train and comes with virtually no warning. It's more bearable with the Ride Armor...What's that? You didn't know that you could take it with you? Too bad! And you don't have access to it at all in the Boss Rush, so good luck with that.
 * . He comes after That One Level, and is fought directly after Sigma, who's hard enough to begin with. He has two forms; the first one uses the desperation attacks of the first 8 bosses except some are modified to last longer; when Burn Rooster's attack comes out, it lasts for the duration of the match. In other words, once it happens, you can't use the walls to dodge his attacks. Additionally, he himself is always moving, making it hard to hit him. Once you've drained his health, he enters the second form, which has a multitude of laser attacks (one of which lasts a long time and then comes back in reverse), has no weakness and is difficult to get a bead on. But the real hell comes once his health dips low and he uses Paradise Lost; if you don't kill him in the next 30 seconds, GAME OVER. No continues, nothing; you're done. Oh, and did you know you had to ?

Mega Man Legends

 * There are two times in the game where you are forced to fight from a slow-moving vehicle and defeat several enemies and then a boss fight, while protecting your vehicle from the attacks. If the vehicle is destroyed, it is an automatic Game Over.
 * The first one is from a boat, and you're fighting several submarines and a few Draches, which can shoot torpedoes at your boat. After that, you fight the Balkon Gerät, a giant frog battleship-mecha, which will merrily chop your boat to pieces if you can't dish out damage fast enough.
 * This is made worse later with the Flutter. You have to fend off several Draches, the Gesellschaft itself, and then the Focke-Wulf, Tron's personal attack aircraft. The Focke-Wulf is especially nasty, the attacks that don't hit you WILL hit the Flutter. Unless you're specced for long-range damage output, you're not going to have a fun time.
 * The Karumuna Bash trio, three doglike Reaverbots in the Clozure Woods Sub-gate. Those things are nasty if you're not Crazy Prepared for them. They all attack you at once and they are FAST. Their primary attack is jumping at you, which knocks Mega Man down and is possibly also a Camera Screw if you land right. Their other attack is to breathe fire, which instantly depletes your life meter's shield so you can take MORE damage from their jump attack.
 * Actually, these guys are some of the easiest bosses in the game. Just keep running in a circle while rotating the camera and firing off shots. It takes forever, but you can win without even taking damage.

Mega Man Battle Network 2

 * QuickMan.EXE. The name alone should key you in to how annoying he's going to be. He is invincible at all times except during his attack frames, moves like a spastic monkey on crack, and his attacks are irritating to dodge, because Quick Boomerangs are really, REALLY fast moving It doesn't help that he's the second boss of the game, the first boss was a complete chump, and the next boss AFTER him isn't exactly threatening either, so he's not even really a "Wake-Up Call" Boss... Worse yet, it's entirely possible for the game to be Unwinnable By Mistake here, as you can't leave his area (either to recover HP or improve your moveset) once you enter (but nothing stops saving...)
 * 2 also had Napalm Man V3. He has 2000 HP, actually has moves that are difficult to dodge, and they pack quite a punch. Crystal Dragon Jesus help you if you have a Wood Style.

Mega Man Battle Network 3

 * DrillMan.EXE makes the game a nightmare. He has lower HP than most bosses, but that's because frontal attacks have no effect - his drill is always blocking frontal attacks, so you have to go around the drill with Wide Swords and such. Only a handful of chips can bypass his drill, so good luck having one in advance when he appears!
 * DrillMan.EXE's younger brother BubbleMan.EXE is seventeen times as annoying! The bubble spam, the bubble spam! Aaaaaaaaaah!
 * To elaborate, he never leaves the back row and the center tile on his side is a hole from which spout an infinite number of bubbles, which block shots and home in on you, always moving into your row so as to block your line of fire at Bubbleman. Since they're limited by number on screen if you stand there shooting them they'll just spawn faster. Meanwhile Bubbleman is using his own attacks on poor distracted Mega Man. Luckily the electric-element first boss has a megachip that can hit and stun Bubbleman no matter where he hides.
 * And that's not even taking into account BubbleMan v3, who almost ascends to Goddamned Boss status. You can't even find him unless Frickin' bastard.
 * KingMan is the second hardest boss. He hides in the far back row while his nearly invincible, regenerating chess pieces attack you and block your spaces. If that wasn't annoying enough, if you stay in the back row (the easiest way to avoid his chess pieces), Kingman has a "plan B" attack, where he summons some new chess pieces and permanently steals an entire row of your area. Not even Bass's auras can compare to this guy's cheapness.
 * The mandatory BeastMan Beta battle is friggin' unfair. Beastman moves rather fast around the screen, and will leap into your side of the field to attack (he likes to go behind you). Now, hitting him isn't as hard as it sounds, but he still is fast and deals a lot of damage.
 * Flame Man has two candles on his back row, each of which have about 20 HP or so, and revive after a short amount of time. They have different effects depending on their color, and if even ONE of them is Green, he's invincible. If one's Red, he recovers HP steadily, and if one's yellow, then fire appears in your area and limits your movement range. Even worse, he can have two of the same candle, which means he can be recovering HP AND invincible at the same time. And his attacks are designed to take advantage of this.
 * This is completely trivialized with Area-grab. You see, his attacks were all designed with the idea that you were fighting in a 3x3 area. That extra row means that outside of you standing directly in front of him, he can't even hit you. Granted this doesn't do much for your grade if you can't beat him in time but it pretty much nerfs him from a YMMV to Guide Dang It.
 * Even among Final Boss standards, Alpha is ridiculously tough. For starters, to even fight the guy you need to defeat Bass.EXE who's quite tough in his own right. Then in the fight itself, you need to chip away at Alpha's torso with buster shots before you can actually deal damage via hurting the core, and doing so is frustrating since you constantly need to dodge a constant onslaught from his claws, which while they follow a predictable pattern, they're still likely to nail you if you're too twitchy in the fight. And if that wasn't enough, you're also bombarded by other nasty attacks such as a ludicrously powerful, very hard to avoid laser blast. And the kicker? If you lose to this guy, you get to battle Bass.EXE again before fighting him! Ain't life just swell?!

Mega Man Battle Network 4

 * Duo's Meteo Fist attack, and by extension Nebula Grey's dragon attack can easily make the fight unwinnable if you don't have Air Shoes or a Panel Return handy, since your field is cracked to the point where you can't dodge anything.
 * That's not the only way the fight against Duo can become unwinnable. Thanks to the game's New Game+ structure you'll fight Duo in a "normal" playthrough with more-or-less the chips you'd get in the first third of one of the other games, and when you fight him on a New Game+ he'll have his health scaled up to truly ridiculous levels. A Marathon Boss is fine for a final boss normally, but problematic in the Battle Network series because no matter how long the battle lasts you can ONLY draw thirty chips. If you don't pack your folder with thirty of the most damaging chips/combos in your library (and then consistently hit with them, as they likely won't be the easiest to connect with), or throw too much defense in your folder, you will run out of attacks before he runs out of health. For the third fight you pretty much need a top tier combo folder out of a strategy guide just to deal enough damage, never mind actually fighting him with it. Oh, and if you're relying on drawing just the right combo in the same hand, for a PA or a powered-up megachip, you'll have to fight another boss right before him every time you get unlucky. Or miss.

Mega Man Battle Network 5

 * CloudMan.EXE evidently took cues from Bubbleman.EXE. He stays in the back row, protected by clouds that periodically shoot electricity. The clouds don't have much health, but despite this can take infinite punishment from time-stopping Chips. He can also hide in a cloud while summoning a pillar of clouds that chases you around, and the pillar remains even after Cloudman stops hiding.

Mega Man Network Transmission

 * Fans of the mainstream Battle Network titles would probably expect GutsMan.EXE to be a walk in the park like in those games. Oh boy are they wrong. While GutsMan lumbers around slowly, he attacks deceptively quick, and his main method of attacking is a massive shockwave that does a horrific amount of damage at this point in the game. If you try to slide under him when he closes in on you, he can nail you with a quick GutPunch that also deals a ton of damage. Thanks to that combination of lightning-speed attacking and huge strength, GutsMan is arguably worse that BrightMan when it comes to being a horrifying wake up call boss.
 * BrightMan.EXE, the first boss to put up shields to your attacks. Whereas the previous bosses were dodging and slapping the boss hard enough before they got you, Bright Man will murder you if you try the tactics that worked on the previous bosses and indeed, some of the other bosses you can fight at the same time. And the bosses get craftier, more agile or with more invincibility shielding from there, making Brightman one hell of a "Wake-Up Call" Boss.

Mega Man Zero

 * Aztec Falcon, the first mission boss is almost impossible the first time you fight him. You have no charge attack (unless you spent time grinding it out by going back to the Underground Base; if you're playing on Hard, you're screwed), he isn't vulnerable to combos, and the Z-Buster actually bounces off when the chance to shoot him presents itself.
 * Phantom is spectacularly cheap compared to his fellow Guardians. Three of the Guardians have an elemental weakness which makes beating them fairly simple. Not Phantom. He's neutral, so it takes longer to kill him. Worse, the other Guardians can have most of their attacks interrupted. Only Phantom's dash attack can be interrupted. It is technically possible to interrupt his Doppleganger Spin technique with a charged blow, but for added cheapness Phantom will break the technique and strike you should you close in to attempt it. Speaking of the dash attack, he'll often do it until you hit him, and he charges to your position. If you don't hit him right off the bat, he'll get at least one hit in, maybe more if you don't score a knockback hit. Finally, his Desperation Attack makes him invisible, during which he'll throw kunai at you from random points on the screen. What's worse, he may potentially position himself above the reach of your weapons, or just plain above you. To top this, he has a Kaizo Trap ability in the second battle.
 * Phantom's caveat is that he's nearly impossible if you fight with the Z-Saber, but a total pussycat if you use the Buster. It locks him into his doppleganger attack and interrupts his dashes even at half-charge. On the other hand, he'll instantly counter at every opportunity if Zero tries to go samurai on him.

Mega Man Zero 2

 * Phoenix Magnion is plain ridiculous. He has an arena which spews fireballs from the floor, for starters. You can arrange it so the floor is blocked the first time you fight him, but not the second. What's worse, he dodges every attack. He literally cannot be hit unless he's attacking. And when he dodges, he counterattacks with very wide-area abilities.
 * He's an Expy of M. Bison, which helps explain his difficulty (Of course). That huge fireball he "shoots" after dodging backwards is the Psycho Crusher. Hit the fireball (with him inside) with a thunder attack(It's your only chance of winning in Hard Mode!).
 * Also, Phoenix is purely reactive, which means that if Zero just closes the distance and waits for a second then he'll begin an attack without dodging or teleporting and leave himself wide open.
 * Kuwagust Anchus on Hard Mode is just insane! Not only is he really fast and powerful (two hits = dead, period), but he's the boss of That One Level! And he has an attack which can't be dodged, meaning you have to get him to change his flight path. And the only way to do this is to either shoot him with the buster enough times, which takes better timing than God to pull off, or deflect him with the Chain Rod. As if you're just supposed to know this out of hand. And he'll keep using this attack till you die.

Mega Man Zero 4

 * Kraft from Zero 4 is a nightmare to fight against. The grenades, the laser rifle that requires you to dash underneath it with split second timing, the giant knife that damages you as it throws you across the arena, the missiles, and the fact that he's one of the few bosses willing to get right directly up in your face on a non-scrolling single-screen battlefield are awful.
 * Making it worse is that you can't revisit stages. Couldn't find Subtanks or enough E-Crystals to upgrade your Cyber Elf? Too bad! Also, he appears after you've beaten 4 out of 8 stages. He also has no weaknesses. Some people were left with no choice but to restart the game.

Mega Man ZX Advent

 * The Spidrill miniboss! It breaks the floor (which, in addition to removing a quarter of your standing space, can affect whether or not you unlock a vital checkpoint so that you never have to do that again), the walls have got spikes on, it summons miniatures of itself, and it has an irritating tendency to spam a move that spins its legs around. The upgraded Spidrill Neo fight in the Quarry is comparatively easy for nixing the first two aspects and coming right after you unlock Model ZX.
 * Chronoforce on Expert starts the fight with Time Bomb, and it never wears off, so he's always in turbo speed. What's more, you know that spreading icicle move he does that you were able to jump between on Normal? Now he shoots two sets of icicles, the second set covering all the angles the first doesn't, making a roughly 90-degree arc that's really tough to avoid whether you try jumping between them or dashing underneath Chronoforce to get past them entirely. And may God help you if you haven't figured out the trick to dodging his attack where he shoots a whole volley of potentially exploding icicles at you from the background, then rewinds them to hit you again.

Mega Man Star Force 2

 * The Infinity bosses, particularly Cancer. They managed to take one of the (arguably) easiest bosses in the game, and upgrade him to have absurd speed and 2800 HP (!). On top of that, his claw attack is now so fast that it's basically an unblockable 200 damage unless you have an Invisible on you.
 * Apollo Flame, in both the second and third games. 2500 HP to start, and that number just keeps climbing. Has a shield that regenerates very quickly and very often, and also has an unavoidable attack that does huge damage, cracks the floor and it's hard to knock him out of it. Also, his weakness (Water cards) doesn't have a lot of heavily-damaging moves - the best ones out there are secret cards, and unless you're really, REALLY lucky, you won't find them.

Mega Man Star Force 3

 * In terms of storyline bosses, Acid Ace in 3. He has a huge repertoire of moves, with something to break any Noise, and he doesn't follow a clear pattern with them. Then there's the matter of his R form, which is fought directly after Dread Joker R without a chance for you to heal.

Mega Man Fangames

 * Rock Man 4 Minus Infinity gets incredibly sadistic near the end when it introduces its hybrid Robot Masters, which are a Robot Master with the AI of another one leading to some nasty combinations. The Cut/Elec Man hybrid is easy enough, but after him is Wood/Quick Man, which combines Wood Man's bulk and tricky-to dodge attacks with Quick Man's agility and unpredictability. Oh shit. It gets even worse however with Needle/Gemini Man. What's the gimmick to his fight, you ask. Oh nothing, you just fight TWO NEEDLE MEN AT ONCE. Needless to say, this is not a fun section in the slightest.
 * The Wily Capsule doesn't let up either. Wily teleports around the screen like crazy and showers you with all sorts of difficult-to dodge projectiles, some home in on you, others explode and fill the screen with bullets ala Touhou. And if that wasn't bad enough, the good doctor has multiple health bars and his hitbox is deceptively small. You need a ton of e-tanks before you even think about surviving this battle.