You need to fully complete this mission and kill Sommers.
The beginning is very linear - you just go down some halls and then fastrope down to the next set of halls. Rinse and repeat until you get captured and Sommers talks to you. When you are free to move again, you'll need to track down Sommers and kill him.
There's no timer so you don't need to rush. Just move forward until you get to the room with the rotating green beams. The beams are instant death, so stay away from them. You need to move up the ramp in front of you, avoiding the rotating green beam, kill any reborn that are up there and then you have to destroy the thing that is shooting the horizontal beam. Squat and move under the horizontal beam so you can shoot down the barrel of the beam emitter in the center of the room. It just takes a shot or two but the weak spot is recessed so you have to shoot right down the beamline. This will lower the shield in the rotating green beam area so you can move over to the next section to rinse and repeat. As you move where the green beams are rotating you'll have to move to the side to let the rotating beams pass. It sometimes feels like they'll hit you but they won't if you are moved all the way over to the side.
When you get all the beams down there will be an explosion and a ramp to the center will appear. On easy you can run up and shoot and melee Sommers until you take a lot of damage and then hide behind a pillar to rest if you need to. On harder difficulties, check out the solution to threat level yellow for a good guide to taking him on. Basically, you back up to higher ground instead of moving forward and shoot his head from a safe spot.