When an enemy is near a ledge, run next to them and press when the button prompt appears to throw them off the ledge. This can be a bit fickle and sometimes doesn’t work unless you’re standing in just the right spot.
An easy place to get this is fairly early in Chris’ Chapter 1 when you’re heading towards the roof of the building. There will be a metal staircase with 2 enemies on the way up. You can run up to them, hit and they’ll almost always go off the ledge. You can then reload the checkpoint and repeat until the achievement unlocks.