I just fought a banshee with a champion who had a lightning hammer equipped. If I just straight up attacked the banshee, it took 10 lightning damage. If I used the crushing blow, my champ would execute his attack animation, and be dazed by the crushing blow, but the banshee would not take any damage. I wouldn't see any floating damage text OR an 'Immune' message.
By design? If so, something should tell me what happened, instead of me wondering why I wasted a turn and did no damage. Otherwise, needs addressed.