Serrated Edge Achievement
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 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.