Right guys. I've put together this guide for you. Hope it helps out.
Landfall – Simply complete the first mission on any difficulty above easy
Holdout – Complete mission 2 on any difficulty above easy
The Road - Complete mission 3 on any difficulty above easy
Assault - Complete mission 4 on any difficulty above easy
Cleansing - Complete mission 5 on any difficulty above easy
Refuge - Complete mission 6 on any difficulty above easy
Last Stand - Complete mission 7 on any difficulty above easy
The Key - Complete mission 8 on any difficulty above easy
Return - Complete mission 9 on any difficulty above easy
Campaign Complete: Normal – Not much advice that I can give here. Normal difficulty shouldn’t be too much of a problem for any gamer. If you want some help tactically then I’m sure people on the forum will be willing to help.
Campaign Complete: Heroic – This won’t be easy for many of you. Unless you’re fairly experienced at this game then I’d recommend co-op with mates.
Campaign Complete: Legendary – Co-op is absolutely essential here. Only the most hardcore of gamers will feel that they can take on everything that the game has to offer, on this difficulty. If you can, then what the hell are you reading this for? You could make your own guide! For those who do wish to go it alone, then the best advice that I (or Ares) can give, is to keep your distance, and do not rush into close combat with anything unless you're 100% sure that you can take it.
The above achievements are all relatively simple. I cannot really give advice on how each mission should be completed but if you do get stuck, then I’m sure that there are members on this forum that are prepared to help
Iron – The “Iron Skull” is located at the very end of the first level, “Sierra 117″. It is located behind the building where Sgt. Johnson is being held captive, up on the roof. You can gain access to the roof on the far right side of the building. Travel all the way over to the left side of the roof, and the Skull will be concealed in the corner.
Black Eye – This Skull is located in the beginning of the second level “Crow’s Nest”, the very 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.
Tough Luck - The “Tough Luck 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.
Catch - 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 - 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 - 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!
Thunderstorm - The “Thunderstorm Skull” can be found on the level, “The Covenant”, after you take command of one of the Hornet vehicles. Fly towards the second force field generator (the one the Arbiter disabled), and land. The skull is waiting at the very top, at the end of the ramp extending over the sea.
Tilt - The “Tilt Skull” is found on the level “Cortana”, in the circular room where Cortana mentions that she “Likes to play games too…”.
It’s a bit difficult to get to, as it requires getting on top of the ceiling of the room. Upon entering the room, head towards the left side where you will see some mushroom shaped extrusions along the wall. Get on top of these, and then look up and towards the right for another set of extrusions. Jump to these. From here, you should be able to jump on top of the ceiling of the room, where the Tilt Skull lies in the centre.
Mythic – This is probably the easiest Golden skull in the game to get. It is located in the very beginning of the final level in Halo 3. Begin by heading into the canyon, but hug the right wall. Very soon, you will notice that there is a turn to the right that is off the normal trail. Head a bit down this path, and the “Mythic Skull” will be sitting there.
All skull locations were taken from Ifti’s Halo 3 Skull Locations - Xbox 360 Forum thread. That’s the place to go if you want details on the skull effects and more discussion on their locations.
UNSC Spartan – Reach 20 Exp in ranked multiplayer. Just keep playing matches and you
will obtain this eventually.
Spartan Officer – Reach 70 Exp, with a minimum of Level 10 skill (in any mode) in ranked multiplayer. Again, all this takes is time. Level 10 skill is not hard to get, but 70 Exp will take a good few days. Luckily, the multiplayer is great fun so it shouldn't really be hard work.
GUIDE CONTINUED HERE