I feel your pain. The boss is called nightmare (not the final boss by the way). Since I am a trophy hunter I had to beat him on Dante Must Die mode. Oh the pain glad I got my platinum already :)

Actually there are 4 difficulty levels. Easy, normal, hard and insane.

I played on normal and had a lot of troubles with the boss. If you want to avoid crazy grinding and in general a lot of headache with the later bosses just play of easy. Although I'm not sure exactly how easy it will exactly be.

So there is no difficulty switch mid game? That sucks. Hate when games do that. 

Ares on God Mode. I got to the clone part and couldn't save my family it was just impossible.

No More Heroes, Jasper Batt Jr.'s second form on Bitter difficulty. At half health, gains an attack that lasts longer than your dodge's invincibility frames, and spams it like there ain't no tomorrow. At 1/4 health, gains ANOTHER attack that is damn near impossible to dodge, and you're lucky if he goes five seconds before spamming these attacks in combination, left and right. And you're fighting on the top floor of a skyscraper, so if he knocks you into a window, insta-death.

I spent almost three hours getting my ass handed to me one day, when finally I did everything right, got him down to less than 10 percent health and tore into his ass in Darkside Mode. The mofo hadn't even touched me yet. Everthing was going PERFECT. So I was going to town on him in Darkside mode, dancing all over his face, smelling my coming victory, when the moment Darkside Mode ended, he punched me clean out of the window in one hit. Like, in one frame, everything I did was undone. I just sat there staring at the screen for 20 minutes before getting up, calmly turning off the Wii and TV, and taking a walk down to the park.

One on Resident Evil 4. That short dude, when he turned into a monster thing.

Demon's Crest (SNES) the real final boss, Dark Demon

Only boss ive failed to beat and still haven't is the last boss in Metal gear rising revengeance. That stupid giant spider thing is ridiculous.

BraLoD said: Yazmat from FF12. Not that I can't beat him, but the battle takes SOOOOO LONG, that I stoped it halfway and couldn't want to get back to it again, lol, that 50M HP bastard

But the great thing about that fight is that if you set your gambits right it'll play for you while you do something else.  Had a nice lunch and watched a movie then cleaned up the last half hour or so to make sure everything went right.

My most hated final fantasy boss would have to be Ozma from 9, too many variables and luck could play far too much of a role in that damned fight.

Byakuren from touhou 12.

I could never beat her in easy mode, i ended up giving up.