Very simple trophy. Full disclosure, you must be playing as a Siren 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 wound the enemy beforehand)
Step 4: Complete action skill (don't wait for it to end, just punch to kill)
Repeat steps 2 through 4 until you unlock the trophy. The Siren is one of the harder characters to unlock this so it will take longer than others.
Happy Hunting :)