Proxima -> Physical Attack characters. Best choices include DP3, Cap3, BP3, QS etc.
Black Dwarf (Cull Obsidian) -> Energy Attack characters. best choices include SW, Cable, Thor3, Psylocke, Emma, Dr Strange, perhaps Luna and Sharon Rogers after the next update.
Corvus Glaive -> Anybody that can negate his high dodge.
Ebony Maw -> Energy Attack characters with elemental damage (fire, mind, Lightning, cold, poison). Best choices include SW, Psylocke, Emma, Jean Grey?, Thor3, etc. perhaps Luna after the next update.