You should be fine with those levels. Just make sure Rosa has time to cast Cure 4 after every Big Bang and that she survives the attack herself. If you're playing FF4 Hard Type (fan-translated for SNES or the PSX version), (SPOILER)Zeromus uses Big Bang very frequently, so if you aren't fast enough to counter it with Cure 4 every time, you may want to lower your battle speed a bit.