This is a pretty tricky skull to explain but I'll try my best. Okay the fog skull is on the level [b]Floodgate[/b]. It takes away the whole radar so you don't know what's behind you. To me I think this is okay considering you can go on one path most of the game and not have anything pop up behind you. Anyways, onto finding the skull.
Take out a battle rifle/carbine (just a long range type weapon or something that isn't automatic).