Or just poison the bastard, summon an aeon, use overdrive, he takes damage, mortibody dies and steals HP, seymour casts dismiss and takes poison damage, repeat, by the time you pull Bahamut, hes probably already dead.

You're going to LOVE fighting Yunalesca. Seriosuly.
*puts foot in TV* Or, bring out Rikku, Wakka and Tidus, Trio of 9999, Blitz Ace and Attack Reels, then at the third form, swap ou someone from Yuna, and blast her with overdrives Yuna NEEDS ZOMBIE PROOF.