I would recommend using the Crystal Sword, as it does have better stats than the Excalibur and by obtaining the Crystal Sword you can then have Edge throw the Excalibur at Zeromus, which will deal massive damage.

I would probably make sure that Rydia's health is above 2000 hp's, as Big Bang can take off damage somewhere between 1800-2500.

The last time I finished the game I had most of my party members at level 53 or 54. I still had a bit trouble defeating Zeromus, but I found the following strategy to be somewhat decent.

Cecil: Attack every single round. Cure someone with an elixer occasionally, especially if Rosa or Rydia are running low on magic.

Kain: Jump. Hopefully he'll avoid Big Bang.

Edge: Throw everything you've got at him(Excalibur being the most effective). If you run out of items to throw, have Edge use elixers on the other members instead of Cecil.

Rosa: Cast Cure4 every single round.

Rydia: Summon Ashura every single round. If you're risky, occasionally summon Bahamut.

Good luck. If this doesn't work level Rydia up until level 60 something when she will learn Meteo. Then the battle is just a joke.