Grunt Birthday Party skull
Level: Arbiter
Difficulty: Legendary
Effect: Head shots cause enemies to explode
At the end of the level you will get a banshee. Fly the banshee to the bottom of the level, and you will see three giant columns go to the middle one and get out of the banshee. You should see a bunch of grunts jumping in a circle around the skull. Get the skull and then shoot all the grunts in the head.

Blind skull
Level: Outskirts
Difficulty: Any
Effect: No HUD
When you first crash you go to the hall and kill the grunt. Stay in that hallway and jump on the light that is sticking out of the wall. Then crouch jump on top of the hall. Look to your left and you will see a ally, go down the ally and you will find the blind skull.

Sputnik skull
Level: Quarantine Zone
Difficulty: Legendary
Effect: Increases explosions from grenades and melee damage
When you start the level look directly behind you and you should see a cave. Go in this and when you come out you will be at at a different place than before. Instead of crossing the bridge across the ravine or whatever go more toward the left. You will see a small a small ledge for you to walk across. It is sort of the shape of your rib bone, when you get to the end of the ledge you will see the sputnik skull floating.