Hello Everyone!
This can be accomplished in ACT 3 Chapter "Making the Grade." Towards the end of the level after the tower defense section and the ogre boss battle, proceed up the extended bleachers to the top of the building. Proceed on top, keeping an eye on the right hand side, until you see a wraparound with a tent on the ground. The Golden Skull collectible is inside the tent. Please see the attached video if you prefer visual representation. Good Luck!