I'm going to take a guess that the "Obtain All XOF Unit Patches" task will involve looking for the patches in hidden locations within Camp Omega rather than on enemy personnel. In the Ground Zeros trailer, Skull Face orders the XOF troops to remove their patches while in the helicopter and has one unit member scatter the collected patches into the wind right above Camp Omega. The achievement could just involve looking for the patches that have scattered across the base, kinda like when you had to look for the little frog guys in MGS3.