the best way i found to beat the mirror was to bang curses right up, and only take with you 2 skills
enfeebling blood and insidious parasite. this will reduce the damage from the "doppleganger" and it will steal x amount of health (depending on your curses level) from him whenever he hits with an attack.
Ideally you want to be in melee range for this so he uses his axe on you.
And yourself, just keep cycling thru the 2 skills, do not attack him, he will kill himself before he kills you.
