Hello Everyone!
This can be accomplished in ACT 1 Chapter "Hell's Bells." After leaving the tunnel and completing the tow truck defense event, you will pass through a tractor trailer and underneath a highway underpass. Continue into the forest sticking to the left side of the level and you will come upon a rather obscure treeline through the fog. Climb on top of the outcropping of rocks and the Golden Skull collectible is right next to the second BIG tree. Please see the attached video if you prefer visual representation. Good Luck!