To get this, all the needs to happen is that you throw a weapon which ricochets off of a wall before it hits an enemy and knocks him down.
This will likely happen by accident eventually but if not, one of the easiest spots in the game is on Chapter 2. When you get there, clear the first floor and you'll have your choice of two sets of stairs to get to the second floor. Take the stairs on the left and the first enemy you see will be in a small rectangular room. If you have trouble intentionally hitting the wall and then him by aiming, use the lock on feature but wait until he's walking away from you to do it. It may be possible that he'll walk a step or two further and the weapon will bounce off the wall and hit him.
Alternatively, this can also be done on the prologue level. After clearing the first floor of the train station, go down the stairs. Take out the first enemy and take his weapon. Below your screen will be two enemies and one of them will be pacing. The other will be standing still. Throw the weapon you just got through the doorway in such a way that it will hit the stationary enemy to get this Trophy.