Location: The skull is in the first outdoor area you come to. Clear the area of enemies, and then make your way to the platform where the door to the next area is. While looking out at the area below, turn left and jump onto the rock there. From the edge of this rock, leap over to the curved rock across from it. Climb up onto the rock wall that this rock is attached to and jump over to the wall. The skull is on a platform at the other end of this walkway.
With this skull active, enemies fire at a faster rate. This effect remains active until you turn off your Xbox and reload the game. To re-activate this effect afterwards, you’ll have to collect the skull again.