Halo 3 Skulls Tips

Iron
Level: Sierra 117
Directions: This skull is at the end of the level. It is located on the roof of the building to the left of the building where Sergeant Johnson is being held captive, if you're facing Johnson. You can gain access to the roof by jumping onto the pipes. Travel all the way over to the left side of the roof, and the skull is concealed in the corner.
Difficulty: Normal, Heroic, Legendary
Achievement Icon: A skull marked with a stylized drawing of a hammer
Effect: When playing co-op, if either player dies the game restarts you at your last checkpoint, similar to Halo 2 on Legendary. On solo, it restarts the mission
Point Multiplier: 3x
Black Eye
Level: Crow's Nest
Directions: It's in the same room you start in. Hanging down from the ceiling is some piping. On top of this piping, on the side nearest to the large display (the one that Hood appears on) lies the “Black Eye Skull”. You can gain access to the pipes a few different ways, one of the easiest however is to jump on the crate like thing, then jumping onto pipes from there.
Difficulty: Normal, Heroic, Legendary
Achievement Icon: Skull marked with a bruise under one eye.
Effect: Your shield does not charge normally. To charge your shields you must hit an enemy with a melee attack (anything but the Energy Sword).
Point Multiplier: 1.5x
Extra: At the screen where you can turn the skulls on, this skull has a black 'bag' under the left eye hence 'Black Eye'.
Tough Luck
Level: Tsavo Highway
Directions: At the point in the level when the huge covenant cruiser flies overhead (after the first shield blocking the road), on your left is some large piping following the road you are walking on. Underneath this piping are some support beams that extend out over a cliff. Jump onto the first of these beams, and travel to the end of it. Look to your left, and the Tough Luck Skull will be sitting on a rock extending from a cliff edge.
Difficulty: Normal, Heroic, Legendary
Achievement Icon: A skull marked with three clovers.
Effect: Enemies always make every chance to dodge and avoid your grenades.
Point Multiplier: 1.5x
Catch
Level: The Storm
Directions: In the first outdoor area with the two AA Wraiths, the skull is on top of the silo over by the ramp that leads to the next area of the level. Destroying the AA Wraiths will cause the skull to disappear. You can use a vehicle to get a boost to the top of the silo or jump on another player's head in Co-op.
Difficulty: Normal, Heroic, Legendary
Achievement Icon: Skull marked with a stylized drawing of a plasma grenade.
Effect: AI will toss more grenades.
Point Multiplier: 1.5x
Catch Skull
Catch Skull Effect: Your Enemies throw LOTS of grenades.
Catch Skull Location:
The “Catch Skull” can be found on the fourth level, in the first area where you need to destroy the Wraith Tanks. When you enter this area, do not eliminate the Wraith Tanks (The skull appears when the tanks appear, and disappears when the tanks are destroyed). The Skull is located on top of the circular building next to the Anti-Air Tank. You can access the roof using a grenade jump, or jumping on the back of a hog.
Fog Skull
Fog Skull Effect: “You’ll miss those eyes in the back of your head.” You lose your motion sensor.
Fog Skull Location: The Fog Skull is found on the level “Floodgate“, at the very beginning of the level. As you are walking down from the anti-air gun you destroyed in the previous mission, you encounter a ramp (next to a missile launcher). Around this ramp, you hit a checkpoint. At this point, you should also hear a marine yelling, “There! Over There!”.
You have a small window of opportunity at this point, but it is fairly easy to get the Fog Skull. Look up, and to the right, directly at the roof of the building next to the missle launcher. There is a single flood form (not to be mistaken with the two other flood forms jumping in front of you) which is holding the skull. Kill him before he jumps, and he will drop the skull down to the ground where you can retrieve it. Too early, and the skull will get stuck on the roof though, so just time it carefully.
Famine Skull
Famine Skull Effect: Enemies drop 50% of the ammo they usually drop.
Famine Skull Location:
This skull requires a bit of grenade jumping skill/luck.
You can find the “Famine Skull” on the sixth level, near the point where the path you follow branches off into two sections (one leading to the area where you need to let the tanks across the bridge, and the other leading towards the area where you meet up with the frigate). Take the right path, as you would if you were completing the mission, and you’ll notice a structure above you / to the left of you (near the area where you encounter a few ghosts).
You need to scale the rocks to the right of the structure, and then travel across it so that you pass 3 of the large support beams extending into the cliffs. Inside of the fourth support beam, lies the Famine Skull. A well placed grenade should have you up there in no time!
Tough Luck Skull
Tough Luck Skull Effect: Your enemies never back down, always make their saving throws, and always charge you.
Tough Luck Skull Location:
The “Catch Skull” is found on the third level, at the point in the level when the huge covenant cruiser flies overhead. On your left is some large piping following the road you are walking on. Underneath this piping are some support beams that extend out over a cliff. Jump onto the first of these beams, and travel to the end of it. Look to your left, and the Tough Luck Skull will be sitting on a rock extending from a cliff edge.