This achievement can only be earned by playing co op with someone.

Start any level which you will get to fight a Khnum, with your co op partner make Khnum's life go down by using any weapon but don't kill him.After you have done making his life go down and when he is only left with a little life stop attacking him using any weapons such as ,Rocket Launcher,C4,Devastator or the Cannon.

To decapitate a Khnum you will have to use the Sirian Mutilator .You and your partner aim right into his head.Then it will rip off khnum's head and the Headsman achievement will be unlocked.