Do the standard buffs on your characters before battle, as you won't be able to use magic within the battle. Bubble, Faith, Bravery, Lure, Protect(ga), Shell(ga), and Haste. Also any other ones you want to personally throw in there. I usually cast all the buffs on all of the characters, minus Lure which goes on the character I am currently in control of (Ashe or Vaan).

Make sure you have some Chronos Tears, Antidotes, and X-Potions. Set your gambits up to reflect the appropriate uses for them and everything.

Designate one character to heal, one character to attack Zeromus (main, usually the one with Lure) and then one to get rid of all the adds. If it gets too crazy, use a Quickening because your magic points aren't going anywhere else in this battle.

If you can't beat it, level up. 50-52 should be a comfortable range for your guys to be in, but you can accomplish it at 47. I'd just aim a little higher to be safe.


P.S. Make sure you have your best equipment on the person attacking Zeromus. And give him/her a Power Armlet (Balfonheim) to nullify Stop, just so you don't have to worry about the damage ceasing for even a few seconds.