The Ancient Golem is pretty big and damages in a large area. Because of this, it is advised to clear out all enemies from an area, and lure him into the open. It is recommended to stay airborne for most of the fight.
The Golem has two main attacks. The first, being a horizontal lunge with his sword, moving very quickly to one side of the screen. He readies up before he charges, so when you see this, just move up or down out of the way. His second attack is a large sword swing that spawns 3 projectiles that all shoot out at different angles, the direction the Golem is facing. This is the tricky attack to avoid. Try to stay under the Golem for most of the fight, so you can avoid his lunge and projectiles, and you should be alright.
It is recommended to have at least 60 health and do at least 100 damage. Meaning at the minimum you only need to hit the Golem 40 times to kill it.
It is NOT recommended to fight with a lance due to the Golem's range and fast movements.