Here's how I got it -
First, take one of the ghosts parked inside the end of the hallway just inside the end of the building where you come out. Use it to kill all of the enemies in ghosts BUT DON'T DESTROY THEM. (If you want you can ignore this part and kill the enemies in ghosts a different way, but you still have to get into a ghost)
The skull is on top of the round building, and one side of the building is sheltered with a wall that curves halfway around it. Drive one ghost into the gap between the building and the wall, then go back and get all of the other ghosts and pile them all on top of eachother in the gap. It usually takes 3 or maybe 4 ghosts to reach the top of the building.
It might take you a few tries, but it's a relatively simple method I invented myself
