I think Assault Mode is tiered right now. I've defeated Purgatrio/Cocytus, Darklords, Blackwings, pretty much the whole tier list while playing it.
I climbed almost undefeated.
Probably because it relied too much on psi-reflector atm, without any fast way to add it. Even you manage to summon it, it is relatively easy to cancel synchro just by flipping it face down, banish, or anything else.