You have to beat the game (completely) on either Normal, Heroic or Legendary. That means EACH level on the same difficulty, not a couple on Normal and a few on Heroic, or it may not work.

If you have someone that has beaten the game in the pre-described manner, let them sign in first, then go get the skull. When you come to the first anti-air wraith, kill it and all the ghosts, brutes etc... but not the main Wraith tank. The skull spawns with that tank on the level, and destroying the second tank disables the skull. 

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When cities burn and armies turn and flee in disarray,
Cowards will cry "tis best to fly and fight another day."
But warriors know it, in their marrow when they die and fall:
It is better to have fought and lost then ne'er have fought at all.