http://carnage.bungie.org/haloforum/halo.forum.pl?read=1060218
Notable sections:
Killing the big AA guns from long range with vehicles, and using the beams and rock shelf to skip the big refining site area are the two big time savers.
One tip: after using the Warthog to take out the second AA gun, just wait there. You'll get one checkpoint, and then a minute or so later, a second. The second one will come after the Wraiths are gone.
If/when you die, do a save/quit before the game reverts to the last checkpoint. Then resume. Hit pause, and the Challenge is lit up again, so you can complete it.
Edit: beat on all fronts.
When doing the mission solo now, I always take out the AA guns from long range. I can't believe I never thought to before.