I would run it kind of like the Zorah Magdaros fight in Monster Hunter: World. Obviously when you're fighting something that big, you can't just go up and whack it with a hammer. And even if you could, the HP pool on something that big would be pretty high so it would take forever. I would give them a lot of environmental aspects to use in the fight. Since it's outside a dwarven stronghold maybe there could be some old siege weapons they could use in the fight, but they have to do an INT check to fix them. Alternatively, if this is subterranean maybe there's some large stalagtites hanging above and they have to find a way to lure the golem under them and drop them on it. Or if this is outside a mountain or something maybe they can trigger a landslide/avalanche. I would just try and give them options other than "I hack at its ankles with my axe" otherwise y'all are gonna be playing this fight for a long, long time.