Here's my strategy:
Enter the battle with Lulu as your starting three and all aeon's overdrive. Lulu, let her use bio on Seymour, not that thing below him, but the face. This is the moment you use all party member except Yuna, Tidus, and Auron, so everyone will get exp. If you don't do that or afterwards, have Tidus cast hastega or haste on Yuna. Summon any Aeon and let loose with Overdrive, hopefully, you'll be able to do this before Seymour puts someone into zombie and cast full life on them. If he does, use Auron and Tidus to bring them back. After your aeon uses an overdrive, it will kill the thing in front of Seymour and he will revive it with his own hp. But whenever Seymour makes a move, including reviving that thing, the bio will do damage to him. Repeat summoning aeons and using their overdrives and Seymour reviving that thing to have bio take life and you should beat him, depending on how much damage each aeon does in overdrive.
(Spoiler: if you think that's hard, wait till you fight Seymour for the final time!)