Get two choppers and drive them up the rocks where you would usually climb to get the skull.
Drive the chopper so the front wheel touches the gap between the rocks and the bridge/platform.
Take the other chopper and drive it over the 1st chopper with a boost/LT.
Then drive to the rib like thing were the skull is, put the chopper so the front wheel slightly goes over the slant.
Then jump on the front wheel and jump to the skull.