Just had a battle with Yam, he was using Magic Shade Soldier, its close failed 2 out of 3 times. First time it got hit with Sergis ss. Failed. Second time Boomerang Fighter hit it for 30 dmg. Success. Third Time Light Herriat fighter hit it for 40 dmg. Failure.
Maybe for some reason it only activates when its hp=0?
I believe it was a nonshielded Magic Shade Soldier, but honestly I don't remember.