Give Sabin 2 Poison Claws, a Genji Glove, and a BlackBelt (Poison Claws can be bought in Tzen). Have Celes learn Bio from the Shoat esper. Have both Sabin and Celes learn Cure 2 (just in case you get hit hard). Then in the battle have Sabin fight, Celes cast Bio, and cast Cure 2 as necessary. You should be able to trash Phunbaba without too much trouble.