There are a few ways to get enemy heads. The demons with the tentacle arm can be killed with a Splatterkill, giving you their head as a weapon. If you have 'Tear you to Pieces', you can also perform 'Stretchin' Necks' by pressing to grapple the enemy and then pressing and holding until you perform the move. The move 'Power Head Throw' from the Weapons upgrade group is helpful as well towards this achievement. A good place to get head kills is in Phase 4 in the room with the 4 chairs you slam enemies onto. Slam 3 onto chairs, and only 1 enemy will continue to spawn. Use Stretchin' Necks to remove his head, and kill his replacement with the head.