Fought said guy again just now, not worrying about item use as much and regearing for defense/mdef. Stocke 62, Raynie and Marco 60, and after 20 minutes of elegant planning each turn, I beat him. Physical attacks are worthless, go for magic, and don't bother using your third tier bursts as they're not very damaging against these guys. I used turn break when ready, and just unloaded g spells. Stocke was my item whore, healing every turn and topping off mp/curing status effects. Raynie was nuke duty, and Marco was my ace in the hole. Emergency double heal when needed, stat buff/shield seed when not, trans-turn to Raynie when everything was smooth. I'll give a fair bit of advice to anyone who's fighting the master right now (complete with no spoilers).
Do NOT worry about using you super awesome items. If you can beat this guy, the final boss is cake. Make sure you win this fight, cuz what you get is soooooooo worth it. Good luck, and for the record, aht stinks!