To be honest the resistance thing might make it a bit unfun to replay more than once but apart from that I actually liked the fight.
Looks cool. And "looks cool" is a strong argument in my opinion ^^.
Regarding Op suggesstions :
-Weakening his dmg is a huge nope for me. That thing has the size of a building. Pls one shot me.
-Increasing his HP while nerfing resistances during " overheating" sounds cool on paper. I guess spellcasters would struggle a bit less against him. They can still do their share of dmg if i remember correctly with lightning in current build.
