Here is a Mythic guide for Halo 3 by The Tyrant, who had one of the LASO achievements named after him:http://halo.bungie.org/gameplay/halo3_slaso_2.0/
The 2.0 version is supposed to have better strategies, but the videos are in the form of fileshares within Halo 3, so if you don't want to or can't fire up Halo 3 to watch them then it is essentially just a text guide unfortunately. However, you can switch to Version 1.0 at the top right for segmented videos that you can watch on your computer. Then if you have any issues, you can go back and read the 2.0 version.
If you leave a comment as to specific areas you found one version to be more helpful then I will edit the solution. I have used this guy's guide to beat Halo: Reach in 2 player LASO so I can verify that they are very helpful.
PS. When he says Mythic he means Solo without using the save and quit method but the methods still apply to coop.