On the level The Ark, get a grav lift from the small building near where you fought the Scarab.
Continue through the mission until you reach the second sloping hallway (stairway). Find the partitioned risers (platforms) halfway down.
The skull is on the top level. Throw the grav lift on the right side of the hall so that it ends up on the fourth small green dot from the door.
Get a running jump and use the grav-lift to boost you to the top. You will get a checkpoint when you enter the room. When you reach the top, pick up the skull.
There, you got the cowbell skull.
Enjoy!
