This is what you do...whatever you do, don't destroy any of the tanks!!!! otherwise the skull won't be there...not just the purple wraith but leave the anti air tank alone as well. destroy all the ghosts and all other enemies so they are no longer an obstacle. then using a ghost, push a crate to that silo and jump up to the job and it will be there. it's better to have someone playing with you coop style. it does take a while to get this skull but keep at it!