I find that he becomes a lot easier once ash learns the skill 'spectral sever', (happens around lvl 12/13ish) as you can drain him of all his MP within the first few turns. then he'll start whiffing all his spells and you can just whittle down.
I trying that and no luck. His ridiculous amount of damage he does and furthermore that he can attack multiple people at the same with High damage is just unfair, no mater how many potions I buy or how much I heal, he just completely eat my health I just Healed. Than, I have to play the guessing game on should I heal at 60hp and get all my heal taken away in one hit and possible die from the continuous attacks or should I go in for the attack and still die. I think this boss needs some serious tweaking and at least have some sort of trade off because having high damage, high health, and high mp is extremely unfair. But anyways, enough of my ranting. I as of now have to grind to level 20 and come and see can I beat him.