At the beginning of the level Floodgate, proceed along the dirt path towards the city. On the roof of the first building to your right, you will see a piece sticking up with red stripes. Aim for that area, and slowly creep forward. The moment you step on the metal ramp, you radio will say "All squads report." You will see a Flood beast appear on the roof in front of the red-striped wall; he will be holding the skull. Shoot him just as he's jumping (using the battle rifle is recommended), and he will drop the skull onto the street below. RB to pick it up, and you're done. This can be a bit tricky, so if you miss or he drops it on the building, just reload your last checkpoint and try again.