Episode 3, after breaking out the radio station and making your way to the first safe haven, slightly on from there there will be a shack with three light sources. Go inside the shack and an ambush will commence, throw it outside when they show up and you should get 4 at a very minimum. There are plenty of places to get this throughout though, so fear not if you miss out here.
Other opportunities exist later on, better ones as well, just make sure you get them all pretty close to you, hold them off with the torch and then drop a flashbang. BAM! Achievement unlocked.