On the level Cortana, enter the circular type room with all the Flood. Look for a small structural piece next to two archways.
Jump on top if it then go up on the rocks to the top left. Turn to the left and jump up again. Turn 180 degrees and jump to the rocks across from you.
Then, follow the rock that is sticking out and leads up on top of the original circular room. The skull can be found in a pool of blood.
It is a skull of Rex, pick it up, and there you go, the Tilt skull.
Enjoy!
