Get overdrive->ap weapons, set overdrive mode to comrade or stoic, get hit by karma often, and make dying help you out. Magic is free so cast life whenever they die, and run away after he is done sneaking up on you.

The sphere levels should add up quickly, so then use them to make your party better so they're able to hit harder and kill him! Don't waste items for trio of 9999 unless you're doing some sort of challenge game