I beat Grym in the forge, despite not really being prepared - my lead guy was low on HP from fighting the lava elemental, and my guys didn’t really have the right stuff equipped. I took a look at it when it appeared and saw the bludgeoning vulnerability, so I switched my monk MC to equip the club of hill giant strength thinking it would make my flurry of blows really effective even if it didn’t give great damage numbers for itself. Lae’zel had a big hammer equipped already, I think from breaking down a wall or something earlier, so she was all set, and I got Karlach to throw stuff at it (Shadowheart had no spells capable of damaging it prepared, so she did a few points damage with a crossbow and healed the MC a little). I had a bunch of hammers in my inventory, which seemed like a good option for bludgeoning damage while also being things I didn’t mind throwing away. The game doesn’t seem to give you damage estimates for throwing, which is kind of annoying since it does for everything else. Anyway, they seemed to work. Karlach went last, drawing its aggro, and I had her jump to the middle platform with the hammer to draw the enemy there. Which worked, except my MC hit it with an opportunity attack as it was heading over and got the aggro drawn to himself instead so it turned back and hit him and Lae’zel with AoE attacks.
In the second round, Grym was standing just far enough away that my melee characters would have to walk into lava to hit it (as they had done like six times while exploring the area to get here). I had Lae’zel jump behind it and land a few hits, then attempted to do the same with my MC but failed to notice his path preview going into the lava, so he started melting and was downed. Shadowheart I think took another shot with the crossbow and healed Lae’zal, then started heading for the lever for the forge hammer, and Karlach threw some more hammers to do some damage and draw its attention, and also I activated her rage which I keep forgetting to do earlier. On its turn, Grym made it to Karlach (I think Lae’zel didn’t do an opportunity attack for whatever reason or maybe she missed) and did some damage but not all that much.
For round three I had Lae’zel help my MC and heal herself, the MC move out of the lava and heal himself, Karlach jumped out of reach of the hammer, Shadowheart moved over to pull the lever… and couldn’t reach. But then I remembered you can shoot levers, so I did, and the hammer came down leaving Grym with 5HP. And the lava drained away and a bunch of mephits appeared (or maybe they’d appeared slightly earlier, I forget).
Round four I had Karlach throw her last hammer at Grym, expecting to finish it off. Instead she did four damage leaving it with 1HP. I had a look at the combat log and realised Grym was no longer superheated and so it was invulnerable to everything. So where did the 4HP of damage come from? I’m not really sure, but I think it’s extra damage from her strength modifier somehow getting through. So I threw my good old returning spear to finish the job, but missed (70 or 80% hit chance, I think). I didn’t think any of my other characters would be able to deal any damage, so I started attacking the mephits and moving my MC towards the lava release mechanism. Which he didn’t reach, but next round Karlach was able to repeat her mystery damage (maybe it’s from the enraged throw?) and also throw the returning spear into the remaining mephits so the lava wasn’t needed..
I think I’ve done pretty much everything in the underdark now, so just a few more areas to explore topside and I’ll be ready to move on to the next phase of the game.