Declassify Mission: Slow health regeneration resulting in death and lights are out.
As soon as the mission starts, go past the open hatch and go around the corridor to the right. At the end, you'll find a COG tag.
Activate the lightmass missile by activating the console. After the scene, you'll be able to activate the Declassify Mission skull ahead. This will make you fight in the dark with slower health regeneration. Open the door once you're ready.
The room isn't entirely dark. But since it's red, the red wound indicator that will appear in your screen as you take damage will be not immediately visible. Fortunately, there's a lot of cover. Just stay in cover and take the enemies out slowly and surely. There's a Gnasher Sentinel in the middle of the first room. You can deploy it along the corridors where the enemies might flank you. Continue clearing the next rooms and you'll be able to complete this section upon reaching the door in the other side.