Hello Everyone!
This can be accomplished in ACT 3 Chapter "Garden Party." You will need to naturally progress through the entire chapter up until the point in reaching the safe house. Before you enter the safe house head back towards the exit of the Maze at the base of the stairs. Instead of heading left (back towards the way in which you came) go to the right and follow the hedges around the corner. The Golden Skull collectible is on top of the stone statue. Please see the attached video if you prefer visual representation. Good Luck!