@Skyglassknife: Okay, managed to rush him down with a Dex build!
Two things to note for the boss however you beat them:
1. They have a threshold (70% HP I think?) just before they start the full heal. Whittle them down to 70% to be safe unless you've got a one-shot move (like the first form).
2. Always start your rush just after they launch their next attack, so you have the time of their full attack cycle + heal ability to finish them off.
Anyway I maxed out Agility > Intelligence > Body > Perception, left out Strength in Corrupted stage. You get like 30% crit from Acquity which works great with the Dex rush attacks. Start with Specialist Strike and autocast every Dex move to finish them off.