I think I had some trouble with Marluxia II and III? Although for II I just needed to change my deck.
For the Ben Diskin fight the only troublesome part the first time around was the part where you need to do the Reality Shift. Had more trouble with Ansem II tbh
Can't comment on 0.2 tho
Other than that postgame fights and fights with restrictions are harder than the usual final fights
Well, a big part of COM is adjusting your deck to the boss, kinda like side-decking hahahahahaha
But yeah, Marluxia.