Gremlin bombers can be shot to oblivion with sentenza :D
Devilites and the wolver series can be tricky cause of their dodging.
Polyps and rocket puppies are tough cause of YOU dodging.
But I'll say what I'm most annoyed about, not necessarily hard to deal with, as it's only happened like a few times, but pisses me off.
When a freezing lumber comes crashing down on me, freezes me, and then his buddy proceeds to tag team and crash on me again, freezing me.
That said, they're not that difficult to dodge, but when that combo happens, really annoyed me as i recall.
Meh, the retrodes(laser-shooting, slashing, swaggering constructs) Actually make you a tiny, tiny bit slower when you are near them... So that they can hit you with the slashing.
My pet peeve is the Grevers-beryl greavers, they shock and stun.