Pharah's Concussive Blast
ability allows her to shoot a small rocket that knocks enemies back, with a cooldown of 12 seconds.
The best maps to do this on are the neutral objective maps of Nepal, China & Greece, as they are designed to have enemies be knocked off. Just fly up above the battlefield and wait for an enemy to be near an open ledge. Use the blast to push them off and score the achievement.
If you'd like to learn more about Pharah and this achievement, you can view this video: