I want them to look as if they have been stuck in a tomb for the last billion years,
Isn't this basically all 'crons? The Tomb Worlds are apparently well-sealed anyway, so if they've been stuck in a tomb they'd look rather clean...
Now if you wanted them to look like they literally just dug their way out of a tomb, that's something else entirely. Should still be pretty easy to paint.
Do the Head/Legs/Torsos separately from the arms/guns so you can detail the crest. Spray them black, Drybrush them Boltgun metal, then wash them over with devlan mud or very thinned brown in splotches. Then very, very lightly drybrush them with tin bitz in a couple patches. (do this after you paint the chest detail, do this whatever color you want.
Then put some PVA glue on them in certain patches put some dark sand on them, maybe even something to make them look colored in moss. Model some of them to make it looks like they're just climbing out of the cround (like cut off at the knees, supporting itself with the hand not attached to the gun, you'll need to be a little creative).