This page details where to find the Mythic Skull during The Command Spire campaign mission.
The Command Spire Collectibles - Mythic Skull
Once you’ve killed all the enemies in this large production room, you can locate the Mythic Skull in peace. To do this, you’ll need to [Grappleshot](Grappleshot “Grappleshot”) onto one of the moving structures, then onto a nearby ledge and finally, onto a platform which leads to a hidden room. See the gif below to find out exactly what you need to do to reach the Mythic Skull: