I would suggest not watching the YouTube video I provided until you get to Chapter 13 due to setting spoilers.
At the end of Chapter 13 you will come to a point where you must walk up escalators to continue. Instead, wait at the bottom and one of the two enemies at the top of the escalator will pull out a grenade to throw at you. Once he pulls out the pin, shoot him a few times but not enough so he dies, only stun him. The grenade will exploded and you will get two kills. Then, simply reload the checkpoint and repeat until you have the perk and achievement.
Edit: If you voted negative, please explain.