The Fire Demon in Dark Souls. Yes, it's a reskin of the two other demons at the Undead prison, but I am always getting caught on something and that side AoE just about one shots me every time The frontal AoE is easy, but that side AoE is insanely difficult to avoid.
Dark Souls 2's Royal Rat Authority is also my bane, simply because of toxic BS. Four small rats that you need to be able to take down in one move, otherwise, you are pretty much screwed. This fight isn't difficult once you manage to do that, but this would be better without the cheap cheese.