Use the Arbitrator and do not use the sword you find in City Of White Nights. The best team consists of Gil, Serva, and Ganz (for non-human side). Aphelion regains HP; you should be at least level 50 to 60 to take out more HP from him. When you can use Limit Break, do so. He is very difficult. You should also have 30 to 45 Herb Extracts, 20 to 25 Herb Extract DS, 10 to 25 Herb Extract Max, 20 Sages, and 5 to 10 Celestial Nectars. Learn these command from books Use Volty, Circle and Life Song. Do not use any links. It may take 20 to 30 minutes to defeat him. You should also equip Samurai Armor (found at Goo's Armory if you are playing the non-human side at a cost of 60,000 Dagols) for a good defense. Always stay in the back of him -- his moves mostly attack from the front. Allow your party to attack him mostly. If your life is low and it hits you, the game will end. When you get a full Volty gauge, unleash Limit Break.