Sure:

  1. Seymour is not immune to Poison. He'll get 1400 damage per round if you poison him (14000 if Lulu has Hit-Expander, lol).
  2. Before Seymour uses his attack with the swords, his helper always casts dispell on the party. Now you can use Defend with every party member and halve the damage. Another possibility is to constantly silence Seymour's helper and stay in Protect-status, which also halves damage, but makes it quite impossible to predict when he does the attack.
  3. Cure the Zombie-status (Wantz sells holy water) before Seymour kills the character.
  4. To halve the damage from "Absolute Destruction", cast Shell on your party - every single hit from that attack is magical.
  5. You don't have to use aeons, for Seymour will instantly banish them, but you can unleash their overdrives.
Once you know a good strategy, Seymour isn't really hard at all.