The following skull can be picked up in Forge mode on the map Longshore
Follow the edge of the map closest to the water until you go up a ramp and are facing a garage door. At this point, go into forge mode, ascend and go to the left of this door, to find the skull behind the vent running up the side of the building, near the bottom.
Exit forge mode and make sure you pick up the skull.
It can be seen in the video below, and I have included a timestamp of its exact location:
] - Skull 6 - Longshore