I don't mind a puzzle element in combat sometimes but the balance isn't right.
You don't need to throw all the D&D rules in the trash with some encounters just so you can let players solo them with exploits. Grym is just like that - total immunity or a devastating weakness. No grey area for D&D rules to matter. Puzzles should be about enabling some advantage or disadvantage, not about being the only factor.
It's stupid Grym would have such a glaring achilles heel weakness in his own lair he was created to protect. If he was anywhere else he would be completely unbeatable as you wouldn't be able to sink him in lava. It's ntirely backwards.
And the whole idea of a level 4 party being in such a legendary place, fighting legendary adamantine golems and crafting lackluster equipment using a legendary forge for reward feels completely off.
I can assure you: I have used web, grease and ray of frost, mirror image, misty step and magic missile to win the fight. All based on 5e rules.
Well, what if he is powered by the very same lava and activated by the very same machine? The golem seems to have some control on the flow of lava and some defects in the system are possible after so long time. In addition, lava can cut both ways and to a less experienced or organized party, it is a death sentence. I agree the golem is stupid but intelligence has never been once of it selling points. My point is I do not think Larian should spent (much) more efforts on clarifying the lore around the golem. It' is not totally absurd (to me) and I am more interested into the overall story arc and the character development. He is after all... nothing but an optional boss.
The equipment is not lackluster. That's more than the average equipment an army will get.
