you have to get rikku to steal from the two guados, so they cant keep topping themslves (or seymour) up with potions. then take out the guados which shouldnt be a problem. seymour uses fire, ice, lightning and water in that order, so you'll need to keep yuna in all the time to do the Nul whatever. once seymour brings out anima, summon shiva (or the one whose name is '???'), and she should be able to do quite a lot of damage. she might not be able to kill him, but you can get yuna to summon other aeons to help. once anima is gone it should be easy enough to kill seymour, just make you keep track of the magic seymour uses, and get yuna to charge the appropriate nul.