Golem applies debuff on the last char who did damage to it to follow them around and hit in melee, even if char in question is far out of its movement range. As far as I understand, you are supposed to lure golem under the press/hummer thing in the center to use level and one shot it with it.
But the whole thing was a bit bugged, since
- Grym suddenly got eerily smart and was avoiding the Cruciblein the Adamantine Forge. With a nervous laugh, we dumbed him back down a little.
Personally I just whacked it to death with my hasted fighter while CC-ing it with Halt Command. Spell only failed once, so I made sure that my far-away archer did the last bit of damage before golem's turn so it wasted turn walking toward him.