Instructions
Begin playing with Yuna, Auron, and Tidus, with Auron attacking the Mortiorchis, which will tap heath from Seymour. Use Tidus and Yuna to cast Hastega and Protect, respectively. Protect will block half of the 3000 hit point Cross Cleave that Seymour often uses.
Utilize the item Holy Water in order to diffuse Seymour's Lance of Athropy and Full-Life to kill one of your characters. Have all of your characters firing off Overdrives as quickly as possible.
Check to see if Seymour is down to 20,000 or fewer hit points. Select your most healthy characters for the ensuing battle.
Survive the Total Annihilation attack from the Mortiorchis. Hit Seymour directly after this attack, attempting to avoid Flare as you do so.
Use Mega-Potion to heal your characters, and summon Bahamut with Overdriven Yuna. Use Mega-Flare to eventually kill Seymour.