BG3 - Adamantine Forge - Grym Battle

Tips:
Maintain your tank in the middle as Prime Target (usually the last that hits).
Be sure to bring Grym into the magma to maintain "Superheated". Otherwise, he will be immune to all damage as you see in part of this fight.
Keep your characters around the edge so you can pull Grym back and forth into the magma and back to the piston as needed.
Bring him to the middle and use that piston for huge damage!