I can name a few off
my nightmares the top of my head...
Archers of Anor Londo (Dark Souls): There's one spot in the Anor Londo area that had me stuck for weeks the first time around. You had to walk on a ridiculously narrow beam while 2 armored archers shoot Dragonslayer arrows (aka super-sized arrows) at you. Getting hit will obviously knock you off the beam, but blocking the arrows is out of the question; because the knockback is enough to push you off to your demise as well. Timing your run is the most crucial factor to beating this spot, and the timing is very strict. Sometimes even the camera can get in the way and send you running blindly X_X
Legendary difficulty (Halo 4, especially): Even with allies by your side, every enemy will focus their fire on you (well you
are Master Chief, but still) The damage you take is overwhelming, so going guns ablaze won't do you good, and avoiding them is just as difficult.
Sirius' Gold Cards (BomberMan 64): The final boss itself isn't that hard (it's actually pretty fun when you play it normally), but if you're trying to get all 120 gold cards in the game, then this is nothing less than a test of patience and luck. In each boss battle, you gain gold cards from performing specific actions (like bouncing a bomb off the boss' head) and the fifth card is always acquired by gathering the other 4 cards under the target time. In the first form, the target time is 10000, which might translate to a minute. This alone is the hardest part about the whole challenge, because Sirius always flies off to perform his laser attack. This attack is tediously long, and he's practically invincible since he's away from your distance. The worst part is that he does it
all the time. It's not like you can use the detonate power-up to make things easier, because you're fighting the very same person who'd hand you that power-up before each prior battle (that traitor! >

) So you're left hoping that he doesn't fly upward before your bombs hit him and win before your target time goes over
Jaquio (NES Ninja Gaiden): This boss has 3 forms, and to this day I've yet to beat his second form. He fires heat-seeking fireballs at you (I never realized how strange that sounded until now ^^; ) while moving back and forth in mid-air, and if you touch him, you get damaged for 3 hit points! And unless you're really good at this game and can get by the entire level (especially 6-2 *cringe*

) without getting hit once, you're gonna have a hard time fighting him; because your health does not restore at the beginning of the fight. The worst part? You die once, you start
all the way back at the beginning of the level! That's just his second form! Imagine actually beating him and realizing that you still have
1 FORM LEFT! D:
Control on 00 Agent difficulty (Goldeneye 007): Don't ask me why I hate this mission so much, just take my word for it >
The final boss of every fighting game ever: Just,
don't bother...