To kill a lone enemy (or even a group), mark John Cooper and learn the enemy and his movements. Run into his visible area. When he sees you, take the knife and stab him when he gets close enough. Be careful; if he punches you before you stab him you will lose.
During the mission where you are testing Dr McCoy's abilities, place the scarecrow at the sunken boat and place it at the edge of the dock. McCoy will then position the scarecrow, then walk to the end of the dock and raise his hand. The boat will rise out of the water and be deposited on the bank. Cooper will then walk over and say "May the force be with you, young McCoy", a reference to the scene in Star Wars: The Empire Strikes Back where Luke Skywalker uses the force to retrieve his Starfighter from the swamp on Dagobah.
During Doc's training level, reach the part where you have to "double up", which means using the coat as a decoy. Place it on the small dock where you shot the boat from and watch. You will see the screen scroll across to where you shot the boat. Suddenly the boat will rise and hover, and a frog will jump off. Later on, John Cooper will walk up to you and say "May the force be with you".