Big Bang shouldn't be one-shotting the party at those levels (except for maybe Rydia). You'll still need to keep Curaja on everyone up with Rosa at all times though, that's basically all she should do the entire battle.

Zeromus counters spells like Flare or Bahamut with his own Flare, and Holy with Tornado, so watch out. If you just keep your party healed constantly you shouldn't have a problem at your levels. Try turning down the battle speed if you're still having trouble.