Change your overdrive mode to Warrior.

First have Rikku to mix Wings to Discovery and Wings to Discovery, that will give you 9999 damage, change her with Tidus. Start with Hastega on Lulu and Wakka. Get Lulu to Doublecast Fire Magic, While Wakka use Sleep Attack. By fourth turn of your team, Wakka have already do 3 9999 damage while Lulu have already done 6 9999 damage. If you get Lulu's Overdrive full, Use Fire Fury to finish him off. You can also use Attack Reels, Aeons overdrives and Slice & Dice.

If any of the above can't help you, summon Magus Sisters. Razzia, Passado and Camisade can do better damage, plus they have Break Damage Limit ability automatically. Don't worry about Sin's attack, Sin can hurt you but Cindy can heal you back.

If this don't help, go and train Yuna's Strength, all of your Aeons strength will increase when Yuna's Strength increase thus making Yuna a valuable asset in FFX.