Originally Posted by
Marshall Banana
This strategy should be doable for anyone who's been casually stealing throughout the game.
It involves taking advantage of Seymour's vulnerability to Poison and Silence. The first few turns are spent casting Poison on Seymour ASAP and speeding everyone up via Hastega (I used Tidus for this) and/or Super Mighty G (one of Rikku's Overdrives which casts Haste, Shell, Protect, and Regen, I believe, on the entire party). I also used Wakka's Silence Attack ability to prevent Seymour from using Dispel on the party. I used Cheer several times with Tidus to help protect the party in case Cross Cleave was used, although, in this battle, it wasn't. Aside from that, I used mostly gems in order to attack both Seymour Flux and Mortiorchis.