Mindjack

When you see an enemy with the "Mind Slave" prompt above their heads after they have been shot or beaten to injured status you do a melee attack to finish them off. There is only a short window of a few seconds that you have to do this before they simply collapse on their own. You need to do this 50 total times to earn the achievement.

You can do this anywhere. No need of boosting with anyone and can be earned while playing the campaign itself. When you shoot an enemy, he will be down on his knees ready to be mind slaved. Don't keep shooting too much or else enemy will die instantly. When he is down on his knees run towards him, align enemy exactly in front of you and start bashing to perform melee/finishing move. Though its hard to melee an enemy in this game so combine this achievement with the following achievement to make your life much easier.
Have a look at the solution for this achievement.