Invulnerable mob ES during Trial of Chaos

Playing the Trial of Chaos, on the 9th round, which was kill mobs within the circles, I selected monsters have ES. The trash mobs (the ones with the sticks, either Chaos Zealot or Bloodrite Guard) received energy shields that couldn't be damaged. Other mobs died like normal. Eventually I died to being swarmed by continuously spawning invulnerable mobs.