This Seeking Tech guide will explain how you can earn the You Dropped This trophy in Ghost of Yotei. In order to do so, check out the following video and text guides:
Step 1: Make sure you learn the Onryo Strike attack.
Step 2: Use Onryo Strike to disarm the enemy. This attack requires three spirit to be used.
Step 3: Pick up the weapon dropped by the enemy and then throw it back at him.
The trophy should unlock if you managed to kill the enemy with the throw.