Ok so I cannot remember what the map is called but its within the first 4 levels.  If you haven't been there yet there is a bomb that the covenant disarmed that you must re-arm.  That room has the skull in it.  The Bomb so you know is a big white thing.  Go up all the stares in that room until you are by the door at the top.  Turn around and you will see a mesh fence.  Right on the right of the fence is a beam that goes straight up and over the room.   Jump onto that and then if you look up to the right you will see a big round pipe that is being suspended from the ceiling.  Jump crouch to that and walk to the end.  Wala another skull to add to the collection.

I looked at my achevments and found it was the Black Eye Skull