In the GBA version I too had problems with Zeromus and his Big Bang, even with my entire party above lv 60. So I lowered the battle speed, that's a first thing that can help. I don't have much to add to what others said. Rosa should cast Curaja every turn.

What I do have to add, is this: if you have Mega-Elixers, don't be afraid to use em. And if you're not afraid of some gambling, scrap Rydia attacking and have her summon Asura to back up Rosa. I found Asura's curaga/curaja whatever she uses much more powerful than Rosa's curaja and it did save my sorry behind plenty of times during that battle. The problem is that what Asura casts is random. But you can always hope for the best, right?

Besides, Rydia not attacking is not that big of a deal. Zeromus counters Meteor, Flare or MegaFlare anyway. And Cecil, Kain and Edge are strong enough to burn through Zeromus' HP by themselves.