Phantasmal Weapon

[Mysterio]

I searched the forums for Phantasmal Weapon, but couldn't find the answer to my question. Could someone please explain how a phantasmal weapon works? My 30 Phantasmal Warriors + PD engaged a Mother of Avalon and 17 Knights, and didn't manage to kill a single Knight. Which units are vulnerable to that weapon? Surprisingly, the Knights had no trouble making short work of them, even without magical weapons.

Thanks!