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:

1. Co-op Distraction - Get a friend or two to goad the Knight into attacking them. When the Knight is in the middle of a long animation (ie, throwing a grenade), get behind him and go for the kill.
2. Active Camo - If you have the Active Camo armor ability, use it to get sneaky and approach from behind.
3. Autosentry - The Autosentry tends to be a priority target for the Knight. Similar to the "Co-op Distraction" method, you can get around behind a knight while they're occupied.
4. Plasma Pistol - A fully charged plasma pistol shot from close range makes Knights very, very angry. Circle around behind them while they are venting their anger.
5. The Long Jump - This requires no special armor abilities or weapons, so it's easy to do even at SR1. Get the Knight to stand at the bottom of one of the two ramps near where the artifact is, with you at the top (or vice versa). This will frequently make the Knight jump to the top of the ramp. Sprint past him while he's jumping, then turn, and he'll still be lazily gliding downward. Line up your hit, and enjoy the show.

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:

1. Co-op Distraction - Get a friend or two to goad the Knight into attacking them. When the Knight is in the middle of a long animation (ie, throwing a grenade), get behind him and go for the kill.
2. Active Camo - If you have the Active Camo armor ability, use it to get sneaky and approach from behind.
3. Autosentry - The Autosentry tends to be a priority target for the Knight. Similar to the "Co-op Distraction" method, you can get around behind a knight while they're occupied.
4. Plasma Pistol - A fully charged plasma pistol shot from close range makes Knights very, very angry. Circle around behind them while they are venting their anger.
5. The Long Jump - This requires no special armor abilities or weapons, so it's easy to do even at SR1. Get the Knight to stand at the bottom of one of the two ramps near where the artifact is, with you at the top (or vice versa). This will frequently make the Knight jump to the top of the ramp. Sprint past him while he's jumping, then turn, and he'll still be lazily gliding downward. Line up your hit, and enjoy the show.
6. Promethean Vision - Use the Promethean heat-vision to look around the corner in mission 4 and wait till the knight turns his back to assassinate him. See how it is done here.