The skulls on that level are The nade jump at the first Wraith and after you Destroy the AA turret and start Rally point Alpha Once u get to where u saw a Machine turret Focus to the top of the Buildings and ull see a infected Marine. HE is Carrying a Skull I recommend using a battle Rifle. On this skull u MUST have perfect timing or the skull gets stuck on the roof, behind the fence, or.... u just fail and the infected gets away