Use a weak weapon, such as a pistol, to shoot a Majini in the shin. If it bends over holding its knee, run up to it until you see the command "Neck Breaker". Note: With Sheva the command will be "Throat Slit". As soon as you see this command, press SQUARE to result in an instant kill with one shot of the pistol. You can do this with multiple characters in Mercenaries mode for a one hit kill.