This can be done on any survival map, but is imperative that you do it on one of the first four maps, preferably Resistance. Once you see the Juggernaut exit the smoke, hit him with two full clips of an MP5, throw a flashbang at him, and then knife him until he dies (make sure not to knife him from the same direction twice, or he will easily kill you. This can also be done on multiplayer. Each time you and the second player attack a Juggernaut, let the other guy shoot him while you get on the other side and knife him until he falls dead. This may not work the first time, but it should work after a couple times of trying.
 

Serrated Edge Video Guide