I tired the others I just couldn't do them right, especially the Grenade Jump. What I did was I took 3 Ghosts, and rammed them between the back of the silo, and the back wall. One was left standing on the front end. So I jumped on top of that one, and then did a crouch jump from there. FINALLY I landed on top of the silo, and nabbed the skull for the cheivo.
Funny though I took the skull over to the other Wraith with me, dropped it, killed the Wraith, and the skull still disappeared.