QUESTION ANSWERED: (9 times out of 10) When the last enemy has less than aan inch of health left, you need to aim your crosshair at the enemy for 2sec2 seconds.(SERIOUSLY TWO F$&@IN' SECS!)seriously. Two whole seconds!) Then "pull your left joystick downward/backward and quickly hold your RT attack button to execute your power move". This WILL ignitiate the cinematic death blow manually and not randomly. This works even better if you successfully counter block & stagger the dying enemy's last power attack.