Dice iirc you just have to kill the bits in the last lavos battle or something.
You have to kill all 3 parts before they revive each other.
The main part can revive both bits and the bits can revive the main part.
They have HPs of about 2000, 10 000 and 20 000.
To be able to do damage to the main part, you need to destroy one bit.
See... it's super easy ;) that or doing this fight more than 50 times just makes it so.