Take Yuna along Auron's path. Summon an aeon (preferably a hidden one like Anima or the Magus Sisters, but it matters not) and attack. That's all I did and killed him in two hits using Anima. Than I went back and fought him again and killed him in two hits using my normal party. Pretty simple, since Yuna's stats indirectly affect her aeons, but in a good way.