The achievement "Knight in White Assassination" requires that you've:
Assassinated a Knight in any Spartan Ops mission.
Knights are not really known for standing still with their backs turned - what's the easiest way to get this achievement?
First of all, an "assassination" is a flashy kill move you get for holding down the melee button when you're standing behind an enemy. If you did it right, the camera will switch to third person for the kill. Therefore, you're going to need to have a knight distracted and vulnerable for this to work.
There are a couple of Spartan Ops unlocked by default that have Knights - Epsiode 1 Chapters 4 and 5. I did this on "Core," which is E1C5. Close to the end of the mission, you'll have two chances to isolate a single Knight - before and after you find the "artifact." Try to leave a Knight carrying the Suppressor, if you can. If you do a melee attack and don't get the assassination, you can always back off and try again as long as you don't finish the Knight off.
Difficulty doesn't matter, so easy is best, although I have helped friends get this on Legendary, so it's certainly possible in other difficulties.
There are several ways to do this:
The Halo 4 Achievement Guide at Xbox360Achievements.org has a video that describes an easy way to do this.
Basically, in Episode 1, Chapter 4, shortly after flipping the first switch a knight will spawn at the end of a corridor. It is pretty easy to sneak up on him and perform an assassination here. If you get seen, just run back around the corner and he will shortly give up and go back to patrolling.