Grym is the name of a titanic, nightmarish, arcane CONSTRUCT that has taken up many days trying to defeat it, that haunted my dreams and almost made me give up trying to get a crafted MITHRIL item AND Grym's GRYMSKULL Helm with each playthrough.
If you do not know, Grym is the BOSS/GUARD of the ANCIENT FORGE found in the UNDERDARK adventures in Baldur's Gate 3.
I had to resort to searching the Internet to find folks who have defeated Grym and all have said to lure the construct under the GREAT HAMMER and then drop it. It works but requires you lure Grym multiple times and he's not too keen on making this easy for you.
Then there are the Fire IMPS that come in after you successfully hit Grym with the HAMMER.
Then you also have to keep the lava flowing so as to be able to do any damage at all!
I finally decided to try obstructing Grym's path and tried boxing him into a lane with hopes of getting him under the HAMMER more frequently and more centered under the hammer to do more damage. This kind of worked, but mostly obstructions would get destroyed and Grym would figure a path through. It was almost impossible to get him under the hammer after the first successful strike.
By trying out other ways of obstructing him I stumbled on a way that not only obstructs him but FREEZES GRYM COMPLETELY! He will NOT move if you manage to place things properly and as long as you keep the lava flowing, you are able to damage Grym until he drops. You can then claim the GRYMSKULL HELM from his body and then claim your Mithril forge item.
Check out the way I have placed obstructions in the video.
I also placed SMOKEPOWDER grenades nearby to do damage but they didn't do any on this recorded try.
Anyway. You will see Grym is TOTALLY FROZEN and will not move at all and my characters just beat him down after that. In this video I made some slip ups and caused myself some damage but if you get it right you should be able to take Grym out and sustain NO DAMAGE.
This works as of PATCH 8.
Good luck. I do hope you find this much easier than using the hammer.