Mission: Cortana

Effect: "What was once resistant is now immunity" -- Enemies have different body parts that may be resistance to certain bullet types

When in the circular type room with all the flood look for a small structure piece next to two archways, jump on top if it and up on the rocks to the top left, turn left and jump up again, then do a 180 and jump to the rocks across from you. After that follow the the rock sticking out and leading up on top of of the original circular room, the skull will be in a pile of blood.