Don't initiate battle with Seymour untill ALL your characters have ovredrive, including Yuna and ALL the Aeons...get Tidus or whoever to cast HASTE on Yuna, or if by some stretch of imagination you have HASTEGA at this point cast that instead, but make sure Yuna has HASTE status either way! Make her summon Aeons in quick time and use all their' overdrives first...this will make the parasite on Seymour lower it's max HP to 1000 by the end of all the Aeons... THEN you shoould use your character overdrives to finish Seymour off. I did this and Seymour died without killing a single one of my characters.