Knight in White Assassination

Knight in White Assassination is an achievement in Halo 4 and Halo: The Master Chief Collection. It is awarded for performing an assassination on a Promethean Knight during any Spartan Ops mission. In Halo 4, the achievement is worth 20 Gamerscore and is represented by the head of a Promethean Knight. In Halo: The Master Chief Collection, it is only worth 5 Gamerscore.

Method
Play S01E01CH04 Sacred, and as one leaves the room with the star map, a Knight will teleport in to the corridor leading back to the main chamber. Stay behind the corner and activate Promethean vision. If the Promethean Knight isn't aware of enemy presence, it will turns its back and face the main chamber. Assassinate it as it walks away. If one failed this attempt, clear up the main chamber and sprint to the main door. The door will close, but the most important part is that a Knight will teleport in, and it will be facing away from the door. If the player is right in front of the door when it closes, an opportunity for assassinating a Knight will be available for the player. Turn around and wait for the Knight to teleport in, as soon as the animation is complete, press and hold the melee button to assassinate it.

If playing co-op, have one distract the Promethean Knight, and when it's in a position where it won't pay attention to others, the player can immediately ambush and assassinate it.