Crystal, when he breathes the Black stuff, Try to jump on his back to avoid em (jump on his tail and walk on his back). Target one of Cerberuses heads and press triangle (doing so will make Donald and Goofy attack. Assign your cur/e/a/agaga spell to X (or Triangle, or Circle) and jump off his back to attack when he bites Donald and Goofy. If you get bitten, keep healing yourself by holding r1 and the button you assigned cure to. Also watch out for his fireballs beacause they deal severe damage and home in on you. Repeat untill hes dead.

Side note: I beat the game today so ask me if you get stuck again.