Finish a Juggernaut with a knife in Special Ops.
A Juggernaut will spawn at wave 10 of a Survival game. The best thing to have on you when one is about is a lot of ammo for an assault rifle or SMG and few flashbangs. Start out by making sure it's just you and the Juggernaut left. Now work around him and just keep hitting him with bullets. To be safe use about 2 clips then stop. From here, find something you can run around so you can get behind him. Throw a flash bang, get behind him and start slashing at him with your knife. If he's taking a fair few slashes, put some more bullets in him. Hopefully after a few slashes of your knife you'll get the kill and achievement. Another, riskier but usually more rewarding, method, is to buy a shotgun, or use an enemy one. Shoot the Juggernaut 2-4 times, then alternate between knifing and shooting. The Juggernaut will either die from a knife or a shot, leaving a good chance that you will slash him down and gain the achievement.