>>717153824 (OP)
>can I come back to it later?
Yes, and the point of no return is at the end of Act 2. There's a message before you enter a swimming pool that tells you there's no going back. Grym isn't too hard anyway.
What you do is stand in the middle, attack Grym, and have
>one teammate at lava wheel
>one teammate at the hammer lever
When Grym walks to the middle (make sure he's superheated), you run the person Grym is chasing and then drop the hammer.