• -Cast Reflect on the front members to bounce Full-Life away, so that his combo doesn’t kill you immediately.
• -Use Bio Black Magic or throw a Poison Fang to make him lose 1440 every turn
• -Silence Grenade (or non-stop Silence Buster) to prevent Protect, Reflect, Dispel and Flare
• -Mix Mighty Guard--two distillers, to bring the frontline under Protect, Shell and Haste (Two Light/Lunar Curtains to give them Regen too).
• I prefer Kimahri's Mighty Guard though, cause I use Kimahri more than Rikku in this fight (due to his Strength-increasing Talk).
• -Cheer x5 to cut Cross Cleaves damage in 1/3. Plus making hard hitters stronger.
• -Focus x5 to make Lu's spells (BOOSTER CACTUAR) devestating and cutting Total Annihilation damages in 1/3.
• -If you're really scared of TA, unleash Bahamut's overdrive when Morti goes in Auto-attack mode.

You can also kill him by Poisoning him (via Bio Black Magic or a Poison Fang) and doing nothing to him anymore after this. He’ll slowly die from Poison and he’ll skip “forms”, so the only things you will see is Lance of Atrophy + Full Life Combo, Dispel and Cross Cleave. No Total Annihalintion.

Answer Courtesy of:

Chris Johns (Tyrant_Wave)