Seymour Flux is arguably the hardest storyline boss in the game next to the final boss. Here's a couple of tips that might help you:

- If you use Bio or a Poison Fang against him, he will be poisoned and take 1400 damage per turn. You can even cast Haste on him thereafter so he takes more damage. Since he and Mortibody can only alternate turns, this doesn't actually speed him up.
- When Mortibody casts Dispel on your party, Seymour's next attack will be Cross Cleave, a devastating group attack. Quickly bring in your tankiest party members and defend with them to halve the damage. You can also put your party in Protect and use Silence against Mortibody to prevent Dispel altogether.
- Once Seymour starts throwing out the Total Annihilations, use Shell. It halves the damage and with an attack as powerful as this, you really want to do that.
- Buy Holy Waters. They cure Zombie. You will want some more of them for later, too.

If all else fails, go back to the Calm Lands and do some sidequesting. There's a cave underneath the bridge inbetween Calm Lands and Mount Gagazet where you can do a quest. Also, you can start capturing fiends for the Monster Arena. Or do some chocobo minigames. Or go to Remiem Temple.