This can be done with a single enemy if you want to get it out of the way.
You can purchase the "Tree Hugger"-mutation as soon as you get Selma as your third teammate. After you bought it select her and go near any enemy, there are some on a hill behind her.
Go near one 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.