I wouldn't exactly recommend using Bahamut since Zeromus counters with Flare. Holy does tremendous damage, but Rosa will be healing too often with Curaja to ever get a chance. Cain and Cecil should be attacking, and administering medicine as needed. Rydia too. (Sure, you can use Meteo, but it takes a long time.) Rosa = Healing with Curaja. Edge = Throwing everything. Seriously.

Too bad FFIVA doesn't have the duping trick though. That sucks. I loved whalloping Zeromus with Excaliburs.

Edit: Don't waste time with Haste/Berserk/other positive statuses. Our good old epitome of hatred will just use Black Hole and negate them.