BladePhoenix

Asssuming this is the seymour at the top of mt. Gagazet, the easiest strategy is have reflect, haste, protect, shell and regen if possible, but definately have reflect in your arsenal of skills, and haste
Make use of the aeons and their overdrives, bahamut first, because it hits him and his parasite thing, which will damage him again, then eventually his parsite thing will power up, and then seymour will cast dispel on a character, then as soon as possible after that cast reflect on that character, seymour will cast flare on that character and deal about 3000-4000 damage, then have your 2 most powerful characters attack seymour himself, and the third either heal or attack the parasite,
Eventually seymour will fall
Protect will reduce the damage from cross-cleave, and reflecting flare should stop it completely as it is a counter attack and he will not attack himself