Keep a LOT of high potions on hand, or x-potions or whatever curatave item in your inventory is best. Cast bio on the keeper. Cast reflect on the keeper AND KEEP CASTING DISPEL ON YOUR PARTY. This will poison him and keep him from healing himself. Yes I have tried this before and no he is not immune to poison. Then use your aeons. If they die, which they shouldn't, unless you're REALLY weak, then you can either dispel his reflect and use magic to finish him off, or just use your most powerful attacks. Also use attack items. This greatly shortens the length of your battle and gives him less time to severely damage you.