This is doable on Dark Carnival < The Barns. When you get to the end of this chapter and you have to fight the horde before the gate will open, continue you down that stretch of alley after the gate until your outside the safe house. At this point, an infinite spawn of common infected will spawn. Once you see a clown leading a wave of zombies, throw a molotov and it should net you the achievement. If not, they will never stop spawning so just keep trying. Here is a video to help:
Credit to EPICgravity for his video.