This can be done with a single enemy if you want to get it out of the way.
"Hog Rush" is one of the first mutations in that skill tree for our pig-friend Borhmin. You can knock down and stun enemy with it. As soon as you have enough skill points (awarded for leveling your character up) to buy it you can get this achievement.
Select Borhmin an go near a enemy without entering it`s awareness-area and save the game. Press X to start an ambush, select the skill and move the cursor above the enemy an press A.
After the short animation just load your save - rinse and repeat nine times.