Well, you can make your trip to Zeromus a bit faster by running away from every battle on your way. Can Rosa usually survive a Big Bang? If she can, you should be fine with her casting Cure4 on every turn. If Zeromus casts Big Bang too frequently, lower the battle speed.

I wouldn't bother timing with Kain's jumps because you still don't have that much time to waste. Don't bother casting Protect, Shell, Haste or anything like that, either, because Zeromus' Black Hole will dispel them. Having Rydia cast Bahamut might not be the best idea since it will often make Zeromus counter with Flare or Bio. Have her use something like Ashura instead. Edge should throw weapons and then attack, and Cecil can either attack or do some emergency healing with spells or items.