There are many way to beat Yu-Yevon:

1. Use Candle of Life which cast Doom to Yu-Yevon.
2. Use Zombie Attack and cast Life to kill him.
3. Get a Celestial Weapon for a character and raise the character's stat, One hit should kill him.

You can't lose the battle with Yu-Yevon and your aeons, as you already have Auto-Life cast.