Level: The Storm
This skull is placed on the silo on the right side of the battlefield. To reach it get the Warthog and ride towards the silo. You'll see an elevated spot on the left side of the silo and that's the place to crash your Warthog in. Jump on the hood and then jump on top of the silo. The skull is yours.
Effect: All enemies and allies are equipped with more grenades and throw them more often.