My Elemental Druid (spams Fireblast and Fissure) cannot damage River Stalker type of enemies. These are big hydra looking things that come out of water in Act 3. All my abilities only target and function on land and this enemy is entirely water-based.
Not massively important, just saying.
My Act 2 Merc can’t hit it either, he just stands there and lets it hit him.