While this can be done with a variety of combos, the easiest I found was to either throw your secondary weapon at enemies when they're leaking sand (aiding towards the Throwing Master trophy as well), or by pressing Triangle and then Square with only one weapon equipped, again while they're leaking sand. This will grab them and decapitate.
There are a few dual wield combos that decapitate as well, but often I'd find myself chopping them in half at the waist instead of the head, whether I was hitting the wrong combo or just timing it wrong, I have no idea, but i found it to be unreliable.
If you need this, you can farm it on the really weak enemies found in a few locations (Near the entrance to mechanical tower, for example). Enemies can be respawned by running far enough away then heading back.