Hello Everyone!
This can be accomplished in ACT 2 Chapter "A Call to Arms." Progress naturally through the level until you destroy the veins to the main nest and defeating the horde that follows. Once defeated, head outside of the parking garage and immediately head to the left hand side of the housing development site. Continue along the left side and follow the path that will wrap around, eventually leading to multiple gardens and a chicken coop. The Golden Skull collectible is inside of the chicken coop. Please see the attached video if you prefer visual representation. Good Luck!