This is a Walkthrough to get Skull #1
On floodgate start from beggining on either Normal, Heroic, or Legendary, plus the skull is near the beggining anyways.
When you spawn you want to be Mastercheif, walk up ahead and use you BattleRifle,look up at a building, and you should see a flood jump, shoot it quickly and it will drop the Fog Skull (I'm sure it is the fog skull) (By Skull #1 I don't mean the 1st level!)
Video Link:http://www.youtube.com/watch?v=nypmB5s60p0