In order to get the achievement you will have to shoot all the pumpkins on the special Cargo Halloween map variant. When a pumpkin is shot it should light up and start to laugh, and the last pumpkin you shoot should say something like "Happy Halloween". All the locations are listed here (map by Karlas).

Once you have unlocked the achievement you'll also unlock a special halloween watch. after getting the achievement you can leave the server peacefully and the watch will be unlocked.