He has a regeneration ability that kicks in at low hp. The cooldown is on the longer end so if you dont get him down before the cooldown is done, he can do it again. Prevent monster heal debuff will prevent him from healing, or dealing more damage to kill him in between the buffs can help.
I have noticed that sometimes the damage absorption stays active longer then it should, I think 2 seperate buffs he has bug each other out sometimes. Making a new game can hopefully fix that if that is the case (next patch I’ve deleted the secondary buffs to prevent this from happening) but that is of little help to you now.
There are several uniques that have the prevent monster heal, but if you dont have any, a good spot to get something with it is the leviathan minibosses in swampy pit, act3. The gloves they might drop have guaranteed prevent monster heal ability. Another cheap way to do a lot of damage fast is using crushing blow aka ‘% reduction to enemy health’.