Level up.
Get Zombie Ward armors.
Cast haste, protect, shell and reflect on all characters.
Use Bio and Silence attack/buster on Seymour.
Dispel Protect and Reflect when he casts them on himself.
Heal after cross cleave ot total annhilation.
Defend before every cross cleave (with protect, damage is 75% less).
Make sure you have shell before every total annhilation.
Use as many aeon overdrives as possible.
Easy method 1: Trio of 9999 + Multi-hit overdrives = Dead Seymour
Easy method 2: Zanmato
Easy method 3: Hastega. Bio on seymour. Use protect, reflect, holy waters, potions to support party. 50 turns later - dead Seymour.