This achievement takes some patience, you may be able to replicate the video or you can use do this on the Finale of "Dark Carnival". On the Finale make your way to the stage, set off all the lights and make sure you pick up a Molotov from the surrounding area. Get onto the stage and stand near the PA system you use to set off the music.
You will hear the horde and if you look to the opposite site of the stage you will see the horde coming up the stairs near the PA speakers. Throw the Molotov behind the speakers and down towards the stairs. Normally there will be a clown running with the group, if this doesn't work keep trying.