This video shows you how to get the skull that is hidden in Bad Piggies Rise and Swine Level 2-IV. The skull is located at the far-right of the level. Use the same contraption as in a normal 3-star attempt, but use the grappling hooks to keep moving rightward instead of drifting back left for the finish line.
If you have a different strategy for completing this level feel free to leave a comment below.