Halo 3
79 Achievements
1,750
48-62h
Fog
Claimed Fog skull.
10
How to unlock the Fog achievement in Halo 3 - Definitive Guide
As soon as the mission starts turn right and follow the path into the city, as soon as you reach the sandbags on the steps aim towards the top of the building on the right side of the street.
One of the Marines will say "All squads report" and then you will see a Flood Combat Form move into sight on the roof. You have to shoot the Flood, making it drop the skull into the street where you can pick it up using RB.
Using the Battle Rifle is recommended so that you can take advantage of the scope, it is also easier to shoot the Flood whilst it is still on the roof as if you wait until it is in midair it may drop the skull on the roof of the other building making it inaccessible.
If you miss it just restart the mission and try again. You can see the entire process in the following video (please note that the video is not mine and all credit for it goes to eliteshadowxx).
Mission: Floodgate
Effect: "You’ll miss those eyes in the back of your head." -- Your motion sensor disappears.
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!”. 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.
Refer to "Mythic" for a video of all the skulls.