Well, the scorpion boss from god of war 3
*Spoilers ahead*
He was acually a really easy boss, but the fact that he exists pisses me off... Here's why, the boss fights in GoD3 go:
Posidon
Hades
Hermes
Helios (kind of)
Hercules
Cronos
Giant scorpion (WTF!!!!!!)
Zeus (Final Boss)
This makes the giant scorpion the second to last boss... yes, you read it right, the second to last boss isn't a god/titan/hero, it's a giant scorpion and then you fight zeus.... WTFFF!!!
But the most frustratingly hard boss for me would have to be the final boss from RE5, don't call me a noob, but he has killed me 13+ times before I could finish his first form. This is because I play online... without a mic, and without giving away any more spoilers, the boss splits you and your ally up, and you have to rely on the other player to save ur ass. I could easily save my ally, but when it came to his turn to help me, HE DIDN'T KNOW WHAT TO DO! A boss that splits you up is fine, but don't make it so that you have to rely on your ally to survive! It doesn't matter how good I am, because my noob allies never shoot him!!!!!! WTF, and then when you get to fight him 2v1 his weakpoint is fking stupid, You have to hit it directly or it does no damage, which means that even if you are an inch off with the rocket launcher he isn't harmed at all!!! Plus there is that glitch where he becomes invincible and auto-kills you, and for some reason my ally ALWAYS seems to know how to trigger it X(