AI no longer flinch from attacks.
Immediately after the armory room that is full of Covenant and human weapons, there is an L-shaped room. Grenade Jump, using a Ghost, up to the rafters of this room, and you will find the Skull over a doorway. Or, if you are playing co-op, get a box from the weapon room, and have one player stand on the box, then have the other player stand on the first player's head, have player two Crouch Jump on to the ledge and grab the skull.
Enemies do not flinch anymore when being hit by ranged or melee damage.