This one is really tricky.
You need to get the killing blow on the Horseless Headless Horsemann, who is a halloween boss. Why this is so difficult is because you are usually competing with up to 23 other players for the kill. In addition, the HHH can kill you in two hits, which increases the difficulty.
Of course, if you can find a server with the plugin, it can be boosted easily, but they seem to be hard to find these days,
Therefore, if doing it legit, you need to find a halloween server with the boss on it.
Some map examples are mann manor, hellstone and carnival of carnage.