u think thats hard, wait till you have to fight seymore omnis later in the game
but here is a quote from the offical FFX strat guide, (hope it helps)
use hatega after u have used all the trigger commands too give u a head start (but it wont last long)
Yuna's protect will greatly reduce seymores cross cleve attack and you can silence buster to prevent seymore dispel and his flare attack
b4 the fight charge up all your aeons overdrives to full and when u get in there use armour break and mental break on him then unleash all ur aeons overdrives one after another, if he's still alive after that tidus provoke is good (can't remember if seymors immune to it) cause he will always attack tidus and take half his hitpoints off but even when u get down to 1 hitpoint he attacks wont kill you.
Good luck, and hope this is of some use