here is an image that shows where the skull is located
http://www.coolbh.com/schnicker/images/Skulls/catch-skull.jpg
and here is a description:
what you have to do is get to the first river bed. Whatever you do, do NOT destroy the ordinary wraith on the ledge. Go to the concrete structure near the AA wraith (destroy that one), and do a grenade jump to the top using a FRAG grenade, and nothing more. Get to the top, and there should be an Assault Rifle, with the skull next to it.
follow exactly what i said. DO NOT destroy the regular wraith...but DESTROY the AA Wraith. ONLY after you do those, then you can grenade jump up and grab the skull located in the picture above.
hope i helped