Difficulty: Legendary
Level: Metropolis
Word: Catch
Effect: Allies throw alot of plasma grenades.

Right after you get out of the sewers(Tunnel), there is a big area and 2 ghosts, kill every thing first, then go to the middle of it where you see a arc that leads up onto a building, grenade jump by it to get enough height to start climbing it. When you get to the top take a left and hug the wall around the structure and the skull will be there.