There are a few things to getting this achievement.
First you have to earn some skill points to unlock the ability.
It is in the first set of skills.
Second, how to pull it off. When an enemy goes to melee attack you, press B and a direction to dodge the attack. If timed correctly a QTE will appear. You will have 2 circles appear on the screen. When the larger one has shrunk down in the smaller on press Y to do the counter attack.
Third, and the main key to getting this is making sure they are incapacitated. If the enemy falls to the floor and have a Y prompt appear above him, he is incapacitated. If he doesn't you will have to do the counter move again, until the Y prompt appears above him.
Depending on the enemy and how much armour they are wearing will dictate how many counters you will have to do.
Last, now you have incapacitated them, finish them off using a finisher. To do this follow the prompt and press Y. You will get more XP killing them this way so it worth doing.
You can work on this during the main story or after in free roam.
If you are having trouble doing none lethal counters try doing the dodge counter pressing down and left plus B or down and right plus B
I found dodging that way makes you stab them in the knee,
Up and either direction makes you kill them.
Some people have had problems with enemies not spawning once they have finished the game. I recommend you try your hardest to get this done during the story, to try and avoid this problem.