Act 4 Prisoner Red Aura heals him to full from hitting my Reavers

I get the boss to 20% relatively quickly, but I cant one shot him. So in the last phase when he is down to 20% and uses the red Aura, he will hit my Reavers and heal to full. The only way I have to beat him at the moment is weapon swap and drop my minions just to let them get resummoned later. This feels terrible, and while not a bug per say, feels like a bug. I am a Ritualist with Skeletal Reavers/Skeletal Clerics. My damage is not a problem, have melted all bosses up to this point. I had this same issue last league and the only way I beat him with minions was Freeze/Stun/Heavy Stun him so he couldn't cast anything.