The first form of that boss: I just use Lulu and Auron and Yuna to finish it.
Lulu casts DoubleCast (Blizzarga and Blizzarga)
Auron Use Power Break and
Yuna Use Hastega

Because LuLu and Auron does 9999 damage per hit so the total damage is 29997
Just repeat and repeat and he will die soon
Or summon Anima with full overdrive and cast it.

The second form of that boss: This is easy.
Just summon the Magus Sisters and use Overdrive or
Order Sandy to hit him instanly, because Sandy does 99999 damage per hit.

After that, he will be vanish.