The easiest way that you can cut off an enemies head is to use focus mode by pressing and holding and hold the button until the light gets to the tip of the blade. If your enemy is standing upwards, then aim slightly higher using the to aim for their head. If your enemy is crouching to the ground, use the to aim slightly lower for the head. When it says with a bonus, then it means that you must perform a perfect slice. A perfect slice is when you release an attack when in focus mode when the light on your blade is at the tip.