Trying to clear the Chakhir stronghold I ran into an elite (one of the slugs) that was unkillable. After fighting the mini-boss in the lower left corner of the stronghold there was this elite slug left that would not die.
It wasn’t “oh, it’s taking forever to grind it down” I had no other monsters around, three 9-headed hydras and a Blizzard on it, then sent 2 Meteor swarms to pound it and its life bar didn’t budge. I kept up the barrage for over 2 minutes and still no damage.
Fortunately it was slow moving and a teleport and a dash seemed to break aggro so I was able to run away and clear the central area and the dual boss fight to finish the stronghold. But why in Akarat’s name are we back to the unkillable monster bug in Diablo?