Famine

You can find Famine on the the level The Ark, near the point where the path you follow branches off into two sections (one leading to the area where you need to let the tanks across the bridge, and the other leading towards the area where you meet up with the frigate). Take the right path, as you would if you were completing the mission, and you'll notice a structure above you / to the left of you (near the area where you encounter a few ghosts).You need to scale the rocks to the right of the structure, and then travel across it so that you pass 3 of the large support beams extending into the cliffs. Inside of the fourth support beam, lies the Famine Skull. A well placed grenade should have you up there in no time!