For this achievement you must purchase the Light Trap ability under Ranged using the skill points earned during the DLC. Once you've purchase the ability you must hold the
button while in stealth and lure an orc towards the trap. use it on 20 orcs and the achievement will pop. What I did personally was laid down the light trap, ran up to an orc and have him follow me to where I set up the trap. That would be the aggressive tactic for getting this achievement (and the quicker method).