Oh, okay.
Basic stuff I'd say... Make sure Marle is in your party for Haste (so overpowered it's not even funny). Use the Barrier items and stuff you should have collected all through the game. (Cuts down on damage.) Also, hopefully you picked the Safe Helm over the Swallow when you were given the option of a powerful weapon or a powerful defensive item by the Nu in the sealed blue triangle thing next to Medina Village. (Physical damage cut by 1/3.)
If you're using Crono, I think I just had him spam Luminaire every time his turn was available, unless I needed him to revive someone. Make sure he's using the Gold Stud accessory (MP cost -75%). Marle can use the Silver Stud. (MP cost -50%.) My other is usually Frog for a mix of damage and healing, and he can use the Hero Medal. Plus, he can dual-tech Cure 2 with Marle to full-heal everyone at once.
I'm assuming you did all the side-quests to get all the best equipment?
Also, this makes it look super easy. (It actually isn't too difficult once you get the flow of it down.)