This is another achievement which can only be done with two players. A Khnum is a tall grey skinned beast with two horns. In the early levels, it will only observe you. To decapitate one, both players must have a Mutilator and leash it at the same time.
The seventh level of the game, ‘Unearthing the Sun,’ would be the best place to attempt this as there is a boss fight with just one and it will only be you and the Khnum. Both you and your partner should just focus on trying to constantly leash him.
If nothing is happening, make sure you and your partner are leashing at the same time and not letting go of the leash. It would also be wise to attempt this on tourist difficulty as they are able to damage you quite a bit.
If a video could help you, here is one made with two players by 300kupa
(VOLUME WARNING! RIP HEADPHONE USERS!):