Fog Gold Skull Location | Halo 3
Location: From the starting point, climb up the hill and follow the trail down to the city. At the dirt trail's end, as soon as you step onto the ramp, look up at the building to your right. One Flood jumps from this building across to the other side. Kill this enemy and it will drop the ‘Fog' skull. You'll have to kill him just as he starts to jump away from the building and hope that the skull lands nearby. If you miss or the skull lands somewhere out of reach, you can just restart the mission.
This video is from our unofficial game guide. You can view the full guide here