So, imagine two tanks fighting each other in melee, shooting each other with their guns. Both of them are strong, but not against each other. They have a really tough armor, so none can greatly damage the opponent. And it goes on forever.
Well, that's exactly how a fight between a golem and a Reaper King happens.
Wiki says: To tame it, you need brute force like a rock straight into its head.
Billy (tribe of billy) - lvl 2: Will my Journeyman slingshot work? I just spawned here and i was told it eats rocks! I have some in my pocket! I WILL TAME IT