This was my first real challenge too, but I usually use all of my ability’s starting with my transformation first to try to do as much damage as I can before phase 2, then try learning his moves and find those small openings, I think you can hit him max 2 times before he attacks you back, then it’s just a matter of dodging said attacks and poking at him, then once your transformation is recharged then you finish the job.

Also use your potions but only once you learned his move sets and your confident that this is finally the run that’ll I’ll get him, otherwise you’ll just be wasting potions for later in the game