Trepang2

This Achievement is a bit of a pain due to the extra effects that a flashbang has other than the flash and bang.

To get this Achievement it requires the enemy to be killed by the direct impact of a flashbang, not when it sets the person on fire after it impacts and detonates. Both give you hit markers but only the direct impact gives the achievement.

I recommend going to the "Gunnarson Complex" (or any wave-based map with a shop) on the Easy difficulty to attempt this, as you can have easy access to the shop with extra flashbangs. While the first enemies of the first wave are the weakest. They have a little too much health to get the impact to be lethal while another shot is too much and kills them, I recommend waiting for the guys with actual helmets to show up. With a handgun shoot off their helmet and hit them in the legs 1-2 times to lower their health, then directly impact them in the head to get the yellow hit marker to signify it was an impact kill.

I did this on easy by shooting one of the non-helmet guys once in the chest with the AR, and then spammed flashbangs at him.