If you're low level, you'll want to use Yuna's aeon overdrives (if you're high level, you should just lay the smack on him). Make sure you have Anima equipped, and that all your aeons are in overdrive before battle, and that Yuna is too. Get Auron's last skill (I forget what it's called now) where he can give his overdrive bar to someone else, and give that to Yuna as well.

Try and get Yuna's ultimate weapons after winning the chocobo races so that she can break the damage limit with holy once she runs out of Aeon juice. Auron/Tidus/Yuna was the party I beat the game with first time at low level.