You can get this achievement in less than 5 minutes, you will more than likely get this achievement just playing through the campaign however if you still need a few kills then simply load up Chris's campaign select Chapter 1, Stage 1-3 (Rooftops).
Run up the stairs push the 1st enemy off the staircase by pressing
, then simply press
and select "restart game from last checkpoint" repeat for however many kills you need.
This also counts towards the Give A Little Push achievement: