Air Tam Storm deals damage according to how much materia you have equipped, so you can unequip all but the necessary materia (Underwater, KotR, W-Summon, Mime, and whatever healing magic you want, like Final Attack+Phoenix). Multiple Mimes are the easiest way to go, combined with KotR. It'd also be helpful to come into the fight with limit break gauges filled (at least Cloud's for Omnislash).