Cast Haste on Yuna, unleash all of your Aeon's overdrives on him, or, use your most powerful Aeon's overdrives, if he hasn't went down yet, use your most powerful character's overdrives and he should go down!

Thats what I done.. ^^

(if none of that works... Summon Yojimbo. Pray he uses Zanmato. and wallah)