If you’ve already tried those, I’ll *try* to think of something else.. :B
I have no specific key moves to share. I don’t remember that did Shiva do 9999 with Diamond Dust, but it sure killed Seymour’s guardians, which allowed Tidus to unleash attacks with Haste (overdrive mode on Warrior, of course). You should really abuse Tidus, as you know his strength increases when he talks to Seymour.
Stick with Teknowoman‘s suggestion, cast each Nul-Elemental you have (right, all four of them *grins*) and that’s about it.
BUY MANY POTIONS AND ANTI- POTIONS
1. do not use ur aeons until he summons anima..(make sure one of them is at overdrive) Use like auron kimahri and tidus to hit the guards and seymour… have rikku steal from both the guards to stop them from regenerating
2. after that it shouldnt take long to get rid of the guards. Use lulu’s magic on seymour when ever u get the chance until seymour summons anima
3. when seymour summons anima, use your overdrive aeon and the rest of the aeons (how ever many you need) until anima is dead.
4. when its just between you and seymour hit im full thrust but watch out for when he casts magic on one of ur characters so u end up killing each other… from there it should be a breeze..