Instructions
Lower your enemy's health. To behead the enemy, he needs to be near death and has to be killed instantly by the projectile you are going to throw.
Back your opponent against a nearby wall. He needs to be close enough to the wall that your projectile will decapitate him and still have enough room to reach and stick into the wall behind.
Throw a projectile at your opponent. As long as the enemy's health is low, you will decapitate him and pin his head against the wall behind him.