I can't recall right now, but any bullet sponge is a pain in the ass. Some hardcore bosses post-Endgame in Borderlands 2 are godawful that way, and then on top of that are genuinely difficult on their own. I guess it's interesting having to strategize who can endure how long in your team and thus always keep at least one player in the arena.
Also, the arena DLC in Borderlands 1 is a bitch. We never got past the second one.
Someone mentioned the blobs in Metro. Those DO suck. Also trying to do any level pacifist in metro is crazy hard.
uh..Helicopters in parts of Half Life 2?
All AI partners existent in an RTS. And yes, they ARE your enemy.