Life regen enemies that won't die

Ran into a couple buggy life regen enemies in Valley of the Titans when playing coop. No matter what we did, we could not get the life of the enemy to drop below 90% before it totally refilled again. After about 15 minutes just gave up and reset the instance.

Fought a magma armor and life regen enemy, while he had the armour on there was no way to deal dmg, managed to kill him, tho it took 2 minutes.

Oh yeah, the magma armor can be a pain for sure. I think the issue my friends and I were running into relates to how the game scales for coop vs single player. Haven't run into anything unkillable in solo, yet.

I can't seem to kill any life regen rare enemies, I'm playing a grenade build and any time the enemy dodges my grenades, its life just goes back up full in an instant. Had to avoid these enemies on the map or even reset the instance.