Inotia 4 - Boss

[Delmonia]

Hi there!
Here i am, fighting against what seems to be one of the last bosses. It's the transformation of the Doppleganger.
But i can't find a way to kill him. He spawns Knights and Priests (who might Heal him) and each time i manage to down him to 30-40%, he just Heals himself to 60-70%... Even if i concentrate myself on Priests as soon as they appear or the Boss.
Is it a kind of Bug or there is a way to Kill him? The problem is not in Survival but it's just impossible to kill him. I tried for 30 min with lots of Potions but it's still not possible...
I hope that someone killed him and would share his Strategy.