Kill 4 enemies in a mission using a Knife.
This can easily be done on the map Burning Bridges. Only the Special Ops class is equipped with the knife, and most of your reinforcements are Spec Ops on this mission.
When the level starts, quickly switch to the Sniper on the ridge. Use his laser designator to take out the helicopter, and then shoot the enemies posted in the two guard towers in front of the bridge.
Now switch back to one of the Special Ops guys, and take cover by the machine gun nest next to the debris. A lot of enemies will be rushing you from the bridge, so just wait behind this cover until they get close. When they get near, just pop out and rush the closest guy with your knife, then take cover again behind the debris once you start taking fire. Just repeat this process until you've knifed 4 enemies.
You'll keep getting Spec Ops reinforcements, so you'll have more people to switch to if you die. They should be helpful in drawing enemy fire as you run out with your knife as well.
The other option is to go onto the bridge, set the first bomb and then wait by the corner of one of the downed trucks. Setting the bomb will trigger a large enemy spawn from further down the bridge, so if you just wait they will continue to rush you.