You can kill him on 25HC with your gear! If your vengeance runs out in the phase transitions make sure to don't kill the horrors form P1 and only do singletarget dmg on the LK. If the silence of the Spirits kill you kill them first but don't kill the horrors!
It took me about 10 tries for my first kill but it's pretty easy after some practice.
Keep on trying![]()