Spyke20

First, shoot all the lights you can in the corridor, there should be one left on the left.
Use cam jam and go to the right until you are completly in the dark. From there, you can shoot the last light.
After the checkpoint, wait until the last terrorist says to his boss that he is going to kill the clerk. Try grab him before he does that.
After killing the last terrorist, use the door that leads to the place where he was. There is another door ahead, when you pass through it there will appear two guys from inside. Grab the last one, knock him and do the same with the other. You don't need to enter the door where the guys came from.
After that, there is one camera and the next door is bellow there. I'm not sure if you need to shoot the lights in there but if you want, shoot them just to be sure.
There is one objective you need to complete before you can go any further, "Find Soth"
To do that, use the binoculars (R3) and the thermal vision where Grim ask you for. Remember the story of the fake leg? Soth is almost right bellow you.
After completing the objective, use the door behind you. There are two cameras if I remember well. Shoot the lights and enter in the elevator.
I hope this helps you!