Use the following video to do it fastest.
Download the map I created to easily get the skull on bungie.net:http://www.bungie.net/Online/Halo3UserContentDetails.aspx?h3...
Tutorial on how to use the downloaded map:
First you will need to load the map in Forge, and start the game. You will spawn on a blank map, with a bubble shield and teleport in front of you. Pick up the bubble and walk through the teleport, as soon you pass, drop the bubble shield at your feet. At this time, you will be in the middle of the bubble. Take your time and spot the skull along with the other bubble shields. Remember, the auto-gun will only hit you from one direction, so you can actually leave space in between the bubbles, or leave your bubble to get another one, as long as you are not in direct sight of the auto-gun. Make a "pathway" of bubbles all the way to the skull, as shown in the video, pick it up with your melee button (RB default) and you're finished!
(Best one available)