Golem King has all the makings of a good first boss. He is hard enough where a skilled player can theoretically lose, but not so hard a new player has no chance. In fact, Golem King follows an extremely predictable pattern that is only difficult if you get greedy or suck at timing your dodges.
For every fight I strongly suggest using the gauntlets. Not only are they fast and deal decent damage, the charged punch attack has invincibility frames that make players immune to energy based attacks. From there, no player should need potions outside of the last fight and upgrades are helpful, but only important if you want to win without taking damage.
The fight starts with the king slamming his fist, rocks fall in one of the two or three preprogramed patterns, he pushes the player away from his body, the slime arm shoots out and goes from one side to the other, with his arm dropping on you a couple times while a slime may or may not spawn. If this sounds really easy, it’s because the fight isn’t that difficult.
Upon starting the fight, attack Golem King around four or five times and then back off and wait for his attack. Make sure a shadow is not above your head and then proceed to attack him around six or so times and then get ready to avoid the slime arm. After the Golem King fires his arm, it will either go to the left or right and you want to be ready. Dodge it and keep an eye on where it ends up. If it goes to the right, it will return and hit you, where as the left is safe. From here, don’t get greedy and repeat the cycle until he dies.