you should over level 30 and build some ATK+ and DEF+ . This boss is freaky hard , you can die easily . You can call this place is to train your characters and to invest your items and gold . It is good preparation for final dungeons . --- When you're level 255 and can kill gods, what do you need to believe in? by TrenchcoatFlash
i just made a run throu the game again and came up with a setup do easily kill him by letting yuki and ulf have berserk (of course) then give the crystal wall spell to alfina and dahna (this may require you to go egg hunting for a while) when you start the figth start by have everyone use crystal wall or invinceble aura and after that use Seiba's orb and go nuts, keep enough mp to put up the crystal wall again if it wears out and the rest should be easy.
I just beat him about half an hour ago at around 25, so I'd say you're not that low. I did get my ass handed to me the first two times, but, on the third try, I came up with a strategy that made him much easier to deal with.
Basically, I played it safe; Dahna spammed Absolute Zero while everyone else healed/supported/cancelled as needed (and Guarded when they weren't needed). Everyone had the Perception skill and Fire Rings (Ulf had the Ebony Armor on, too) equipped, and I kept Diggin' up at all times. I ended up losing two characters when it went into bird mode, so I used Yoat's Orb, which helped a lot.
I also did some gambling so I could outfit everyone with Ninja Slippers/Mysterious Clogs, which aided my canceling efforts quite a bit. --- Now! This is it! Now is the time to choose! Die and be free of pain or live and fight your sorrow! - Auron, FFX