Very simple trophy. Full disclosure, you must be playing as a Hunter to unlock this trophy
Step 1: Reach level 5 (unlock action skill)
Step 2: Find enemies (skags outside Fyrestone are best)
Step 3: Use action skill (make sure to lock onto an enemy with cross hairs)
Repeat steps 2 and 3 until you unlock the trophy. The Hunter is one of the harder characters to unlock this so it will take longer than others.
Happy Hunting :)