For that part, I usually tell the lads to occupy those sniping positions after they save the ammo crates, and try to set a charge on the SBD dispenser. I find that the Elite bugs show up a few seconds after you start setting the charge on the dispenser. When they do, I disengage, find cover, and try to snipe the Elites before they make mincemeat of Delta Squad.
Hmm, here's some food for thought. Has anyone considered that maybe, just maybe, someone high in the chain of command was trying to get rid of Delta Squad? coughPalpatinecough