You can get this achievement in less than 5 minutes.
Load up Chris's campaign select Chapter 1, Stage 1-3 (Rooftops).
Simply sprint up the stairs push the 1st enemy off the staircase by pressing
, sprint towards the 2nd enemy and repeat, the 2nd enemy may of moved so if this is the case simply press
and select "restart game from last checkpoint" if you move fast towards the 2nd enemy you should have no problems pushing him off the staircase. Keep restarting and repeat the process till you get the achievement.
You will also get the Silent Killer achievement when you push the 1st enemy off if you haven't got it already from playing the campaign, repeat for however many kills you need.
Credit goes too "LetsPlay Community" for the video.