i would leave the fight as is and just add a weapon or two that downgrade one type of immunity to resistance. That would help with itemization as well.
Maybe they could be hard really hard to find runes for a weapons even protected by a puzzle if you want and you have to do side quest to unlock them...
+1
There should be a weapon called "Hot Hammer" in addition to the Anvil method, and the weapon should do just that. It would be alluded to by the Drow explorer's documents in each case.
Edit: Or maybe have a special bludgeoning weapon like that while requiring Grym to get regular "Heated" by fire magic. I still like the puzzle requirement though.
