This sucker is "Holy" based so the only real damage seems to be the one "dark" spell Lilith has. And the Aquavirus spell works okay, but "Jump" and other typically big damage spells are like a joke. And you don't dare use any thunder-based attacks because you will just heal this Holy Terror.
It doesn't seem to be affected by status effects either. It seems like an evil ring might work, but then I would have to backtrack (or even go to a very old save) for the right bread and I'm not even sure it's in the nearby town.
EDIT: Wow, this will push your luck and if you succeed the first time .....
Since LadyJJ is following in my diamond boot tracks and I'm sure she and others seeking the "alternative" ending will be going through this, here is what I did when I finally got down to only the Batadon
Spoiler: show
Of course, you will need either healing potions, bolt jars, and other goodies available.
When you succeed, make sure you talk to the "Z" dude until he gives you
Spoiler: show