Uploading this one for myself. This took many attempts and it's one of the hardest bosses I've fought in any game. I didn't use any cheese, starting cursed relic, or Chaos element Arcana. I figured out my own build and was finally able to defeat his ultimate form.
His Phase 3 is him basically using ~7 attacks in a row, followed by 1-3 seconds of vulnerability, before repeating the cycle. He's super fast, aggressive, and does high damage.
![](https://i.ytimg.com/vi/hUR_JaBw6GU/maxresdefault.jpg)