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 X 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.