Easy to obtain on the 4th level in the game when up on the roof. As you start make your way to the bottom right corner of the dance floor and interact with the electricity box with Y. This will ensure everybody burns and dies who is on that square. You will unlock Murderer
too at the same time if you haven't already popped it plus Hellhound
for getting 20 kills. Below is a video if you'd like to see me getting the achievement.