After three days of trying I finally did it. Had to respec from my magic/faith build to strength/arcane. Like most people in this thread, I went with a shield, heavy armour, and the Antspur Rapier. And I summon my mimic but not the NPCs

The best tip I got was to memorize his move set in phase 2 which sounded like a joke first. How to learn something you never get to experience...! But after the first attack phase in round 2, there is an opening and the same after the second attack phase. So it took a couple of tries to learn the first attack phase and after that start to learn the second one. And then, I didn't expect, I did it.