If you want to "own" Zeromus, here are some hints.

A safe level is 60 or above.

Use Bahamut instead of Meteo (if you have it yet) because it takes less time to Chant.

When everyone is revived but Zeromus is still a big, ugly, blue thing, have everyone but Edge use Parry. Have Edge use Steal every time. Eventually you will get an item called Dark Matter or DrkMatter or something liek that. Having this decreases Big Bang damage.

If you cheap, use the Duplicate Item "glitch" and make 15-20 Excaliber or Crystal Sword/Ragnarocks. Have Edge throw these for some sick damage.

Since all Cecil does is attack, You can try to Beserk him if you want, after he uses the Crystal.

Have Rosa Cast Fast on people when she isn't using Cure3/Cure4.