just charge up all your character and aeon overdrives n let him ave it!

if u hav wakka's attack reels then that shud just about finish him off. i don't know if blitzball has been blocked at that point in the game. but if not i recommend getting attack reels.

if u don't have attack reels then aeon overdrives are the way to go. u can call an aeon in, use the overdrive then seymour will dispatch it, so uve no need to worry about the recovery time! and as aeon overdrives attack everything, mortibody will take health from seymour to recover itself, doing even more damage to seymour.