So, this was actually quite easy, did it first try and everything!
It’s important to understand a few things firstly, Grym can only be damaged when he has the “Superheated” condition, which he gains from being in lava. Secondly, Grym will only go after the most recent person that attacked it. So make sure to turn off attacks of opportunity and similar things. Third of all, his biggest weakness it to bludgeoning damage.
Before the fight I equipped 3 of party with my best bludgeoning weapons, and had them wait by the wheel that activates the lava. I then had a magic caster stand over by the lever that slams the forge hammer.
When Grym appeared, I had the group of 3 hit him with everything I could, and made sure to end with the caster hitting Grym. This makes it so when its Gryms turn, he’ll chase down the caster.
Using this I would alternate between pummelling Grym with the group of 3 & then blasting him with my caster, steadily wearing Grym down till he fell.