I only play on Medium and even then I still sux, but...
Fighter: Dragon, obvious... I refuse to imagine what it's like fighting them on Insane or with an edited AI Profiles table, and it doesn't even look Shivan enough to me...
Bomber: Nahema... whenever I fight that thing it can always burn past me... I can chase it down, but the Loki lacks firepower
Then again I hate them all in general - their strafing lets them turn much faster, and turning my mouse to keep up suddenly becomes an enjoyment-detracting chore... on those rare occasions where I get the game working on Linux, my mouse barely moves, and that's even worse~
And the Aeshma to me gives off that "I'm just a useless crap who's slow and weak and I resemble a giant flying face with two hands and my creators didn't even bother to center me in the Techroom" vibe. (no offense intended, it's just the feel of it.)
Whatever deadly repose or whatever it brings is effectively cancelled out - when I'm really pissed, seeing them is like, "Damn, just STFU and GTFO so that I don't have to waste my time pumping Kayser on your ugly (CENSORED CENSORED CENSORED CENSORED~~~~~)!!!"