Please hide your spoilers next time.
I'm fine, though, but newbies might get mad at your topic title.

Oh well, make sure you equip Yuna and Auron with those Zombie Ward armours that Wantz sells.
Then when Seymour Flux uses Dispel, it means Cross Cleave is coming up next.
At this moment, bring out the chars with highest HP and Defense.

Next threat is Total Annihilation; it is cast right after the message "Seymour watches... and waits...".
Make sure everyone has Shell on them, or else this attack will surely toast you.