Complete Monster: Devil Hulk and the Abomination are not very nice manifestations of the psyche of their hosts. The fact that they are literal monsters doesn't help matters.
Demonic Spiders: The Combat Wardens. They're fast, they can dodge projectiles, they'll smash into you while you're in midair, and they'll generally make your life hell if you're not careful.
Goddamn Bats: The MLRS units always seem to have a missile ready to interrupt all your attempts at pulling off cool combos. Fortunately, you eventually gain the ability to catch missiles and send them right back.