IRON (Gold Skull)
Mission: Sierra 117
Effect: Dying in single player restarts the entire mission. Dying in coop restarts the last checkpoint - no chance for respawning.
Instructions: When you've rescued Sergeant Johnson at the end of the mission, head towards the hovering Pelican. Do not enter, but instead turn left and run up the walkway's outer right edge. Follow the corners around the building until you reach a shadowy dead end. The skull is yours.
BLACK EYE (Gold Skull)
Mission: Crow's Nest
Effect: You must melee enemies to recharge your shield or achieve an overshield.
Instructions: As soon as the mission begins, look up. See the grey pipe running above you, on the left hand side of the room? The skull is on top. To get there, climb the stairs, then use the crates and maroon steel beam as stepping stones to the pipe
TOUGH LUCK (Gold Skull)
Mission: Tsavo Highway
Effect: Enemies always charge, always dive for safety and never flee a fight.
Instructions: Halfway through the level, your Warthog will emerge onto a freeway, the controller will rumble and a Covenant cruiser will fly overhead. Exit the vehicle, jump off the freeway and climb down until you see some support pillars. Leap from pillar to pillar, hugging the left curve of the canyon. You'll eventually find a rocky outcropping with the skull.
CATCH (Gold Skull)
Mission: The Storm
Effect: Enemies have unlimited grenades... and use them OFTEN.
Instructions: Get to the first large area with Ghosts and Wraiths (not the area with the Scarab). The skull is waiting on top of the water tower in the middle. Parking your Warthog on the side of the tower facing the buildings (and away from the cliff) should give you the height necessary. Otherwise, use friends or a grenade jump. NOTE: Make sure the purple Wraith appears in the far corner, but do not destroy it. The skull will disappear if you do.
FOG (Gold Skull)
Effect: Say goodbye to your motion sensing radar.
Instructions: At the start of the mission, as you head away from the hill and into the base, Flood zombies will leap from one rooftop to another. Snipe the closest one in midair and the skull will drop onto the ground. Don't kill it on either rooftop or the skull will remain out of reach. Definitely takes a few attempts, but playing on normal difficulty helps.
FAMINE (Gold Skull)
Mission: The Ark
Effect: Killed enemies drop half the amount of ammo they usually do.
Instructions: Your Warthog will eventually reach a fork in the road, where you can continue forward or turn right. Turn right. You'll find a group of Ghosts and, on the left, a large stone structure. Climb the rocks on the right, jump over to the structure and head for the last support beam. Resting on its most precarious tip is the skull. Grenade jump, stand on a friend or bring one of the Ghosts up for the needed height.
THUNDERSTORM (Gold Skull)
Mission: The Covenant
Effect: All enemies promoted one difficulty rank. Basically, every foe is tougher and smarter.
Instructions: As soon as you get a Hornet (and see the chapter title "If You Want it Done Right") fly around the bend and land on the first structure. Lying inside the part of the building that juts out over the water is the skull. Pretty straightforward.
TILT (Gold Skull)
Effect: Very hard to remove an enemy's armor. You will hate the Brutes.
Instructions: Race through the mission (don't worry about the Flood) until you reach a large flat platform and hear Cortana talk about "games" and how she "likes playing too." Search near the back rock wall for a little metal nub with an arrow pointing up. Begin your ascent there, jumping from ledge to ledge until you reach the flesh-colored pipe. Walk across to the upper platform, where the skull resides.
MYTHIC (Gold Skull)
Effect: Enemies receive double their usual health.
Instructions: Stupid simple. Start the mission, running forward and sticking to the cliff wall on the right. When the light goes dim, flick on your flashlight and look for a tunnel, also on the right. At the end is the skull.
[more cheats for Halo 3 here]