Aim for the neck, and strike with the closest limb. Such as, with your hand, punch him in the neck, turn your hand to grabby, and then rip upwards. Another way is to shove the person's head to the ground, hard.
That guy named Poppo You know, the leader of Fyre.
As poppo said, aim for the neck. If your using your foot try and hit it with the tip of your foot (your toes). This gives the strike more of a cutting/slicing motion.
If you using your fist then swing upward, so basically... hit him in the chin. Like an uppercut type strike.
And last but not least, if your holding a part of his body at the time of going for the decap, pull down. Thus giving you more upward momentum.
Feet seem to have the highest ease and momentum to decap but requires accuracy and percision.
It is also a little tricky as feet are on the ground and the neck is up high on the body.