The following skull can be picked up in Forge mode on the map Assembly
This skull can be found on top of one of the large metal supporting beams that connect to the middle part of the map. Just fly up to the beams, and you should see a skull placed on one of the beams in between "tower side" and one of the bases.
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 1 - Assembly