Yeah, that was a tough fight to learn. I'd say try to get through the dragon phase consistently, then once you can do that, really focus on the Nameless King's moves. He only has a few and they all have an appropriate call/response pacing.

ALSO, Dark Souls 3 is secretly a rhythm game. Put on some headphones and turn up the music. You're dancing with the bosses just as much as you're fighting them.