Well, I used tidus, Yuna and Auron (I've also use rikku's overdrive once)
equip tidus and yuna with a Blessed Shild/Ring.
Put Auron and Tidus' overdrives has Warrior, so that way u'll have yr bar always full, don't forget to put Yuna's overdrive has Healler.
With Tidus put haste on everybody and cast Protect and Shell with Yuna.
Focus your attack at Mortiorchis, and i'll absorve some Hp from Seymour. Keep your characters Hp high cuz Total Annihilation will come when Seymour's Hp starts to decresse! When it's above 8000 u can use Shiva's Overdrive and finish with him
That's what i did anyway.