Amazing idea! I saw someone saying that there should be 3 witches that "make his shields". One make a void shield, one makes a solar shield, and one makes an arc shield. Maybe, you need to kill all the witches to destroy the shields (the witches have void, solar and arc shields unlike all the other witches which are just solar all the time). Once you kill the witch, you can destroy the shield. Once you kill all witches, you destory all the shield, you can hit him. But, he's immune to elemental weapons. Only kinetic damage hits him. Problem is- I've never seen a special or heavy weapon with kinetic. Primary-only boss! Yay!