Use a weak weapon, such as a pistol. Shoot a Majini in the shin. If it bends over holding its knee, then run close beside 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 PlayStation3 or X Xbox 360 and it should 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.