You may need to level up a bit.

Anyway, during the first phase of the Zeromus fight, before he transforms into his final form that uses Big Bang, have Edge steal the Dark Matter from him, this item will make the damage of Big Bang fluctuate so it doesn't kill you every turn. Afterwards, use the Crystal to start the real fight...

Cecil - Should mostly attack but do back up healing and reviving.
Kain - Jump and back up healer with items.
Rosa - Should Haste herself and Rydia, then heal every round with Curaga so you don't blow through your MP. Don't bother with too many buffs cause Zeromus has a move that resets them.
Rydia - should cast Slow, then Bahamut, Flare, or Meteor. I don't know if they altered their MP and casting time like previous ports, but one of these three spells should do.
Edge - Should throw any weapons or Shuriken you have in your inventory. Excalibur, the Kitchen Knife, and the Shuriken will usually do anywhere from 5000-9999hp of damage so don't worry about holding back.

Zeromus will use:
Black Hole - negate any debuff on him and buffs on your party
Bio - Heavy Non-elemental Poison damage, causes the Sap status effect.
Meteor - Oddly enough, only targets one party member for massive non-elemental damage
Big Bang - Massive non-elemental Gravity attack that fluctuates in damage if your party has the Dark Matter item. He will use this attack a lot

Chances are Edge and Rydia will die often, revive Rydia first and only revive Edge if you aren't taking too much damage cause he's a glass cannon, and your party is better off trying to take Zeromus down quickly rather than fight a war of attrition.