I have been trying to kill a few guards with helmets (knights?), and haven't found a way to do it. Knives, drop kills, kill from behind, none of those work. I even tried dueling them, but they just kick me and then kill. Is there any way to kill these guys?

Edit; I found a way and for those who need help with this, if they don't have full plate armor, you CAN kill from behind, if they DO have full plate armor, only thing I found is dropping a crate from a crane on them.