Gear up agent, I have a new mission for you...
Load up the level: "1.1 The Key to Crack the Code" (the level after the prologue) and play through until you are trapped and get picked up by a crane, when you escape there will be two guys.
Kill the first guy and sneak up to the second guy on the ledge and push him off, simply rinse and repeat 4 times by reloading the checkpoint from the start menu.
I've posted a video below for a visual reference and extra guidance.
Credit to 'EziosBladez' & 'Xboxachivementguider' for the videos.
(Update: 18/08/2014): The first video is no longer available so I've posted a new link but unfortunately the video quality is not as good as the previous video.
(Update: 13/08/2016): If the game becomes backwards compatible I will record a new clip with the Xbox DVR, although this achievement isn't difficult anyway,
Good luck agent.