Battle speed is one of the biggest concerns.. there are many bosses, especially Z that are just plain HARD if the battle speed is too fast.

Even with a level 75+ party, I imagine you'd have trouble with maximum battle speed on Zeromus.

Rydia's HP is rather... bad, and I would alternate between Ashura(sp?) and Bahamut, leaning more towards Ashura. Rosa Cure4 every round... it really hurts when he gets two big bangs in between a Cure4.
He shouldn't be as hard then, although he still is pretty hard.