Ninja Redux Achievement in Halo: The Master Chief Collection

In order to unlock this achievement, you must obtain the Beatdown medal at least five times in any Multiplayer map and gametype by melee attacking an enemy from behind. Unlike a frontal assault, a melee attack to the back will kill enemies in one hit. If you're having trouble with it, just make sure to get the drop on unaware enemy players or perform a "ninja jump/beatdown" and this achievement will be yours in no time. Take note that this achievement is cumulative, meaning you do not have to get five assassin medals in a single match.