At the start of the mission, examine the Declassified Skull to start the optional challenge. In this section, heavy dust will reduce visibility. Use TAC/COM (press the down keypad) to track your allies' positions. Keep close to them to increase your survivability. TAC/COM will also allow you to get a brief outline of your surroundings.
In the middle of the section, you'll encounter a large group of maulers, wretchers and even cleavers. Don't get yourself surrounded and use grenades in their midst to soften and take out their ranks. There's also a mulcher (mini-gun) by a crate to the left of the area. You can use this to provide your team extra firepower when dealing with the locusts.
After clearing the area, your team will gather by the exit. Before regrouping with them, check the wall to the right of the stairs leading to the exit. You should find a COG skull there. Go further to the right of that skull to find a corpse with a COG tag.
T. Loomis, 1LT, Halvo Bay
Regroup with your team mates and proceed to the exit to complete the mission.