The achievement description is somewhat misleading, as you do not have to win the event, just finish it.
The Monster Mash event is located in Cranberry Bog, (the southeastern 'red' portion of the map) in the Watoga High School Building on the southeastern side of Watoga City.
Sometimes the event doesn’t appear on the map, but it can still be active at the time. If you’re going for this achievement only I would suggest heading to the location to see if the event is active even if it isn’t shown on the map. If not then switch servers by quitting to the main menu then pressing play. The event should start as soon as you enter the building:
Im not sure if you need to be in a team or if there needs to be more than one person in the building to start the event. I was with a team of two others at the time.
EDIT: Thanks to LoboSinSombra for informing me via comment that you CAN
complete this event solo.
The Monster Mash event is similar to the "Oddball" game mode in Halo. The point of the mission is to compete with other players to wear a clown mask the longest in order to attain the most candy (points). Many Feral Ghouls spawn so I wouldn't suggest trying this event until you're level 15-20; however, if you die while the event is active, you do not fail.
There are three rounds, with each round lasting 5 minutes each. Once all three rounds have concluded, the event will be over and you will earn your achievement.