I personally had a problem with this achievement because everytime i seemed to get close to an enemy i would just get torn down by enemy fire. But i found a really easy way to unlock it.
During a firefight kill every enemy except for 1. Get close to him using cover manoeuvres and run up to him and hit B to knock him down. You now have to option to execute him. DON'T.
Just wait for him to get back up and press B to knock him down again, each knockdown counts towards this achievement.