Singularity: "You think it ends, here, don't you? D'you know why they call it Dark Mirror?"
Gabe Logan: "I don't care, Singularity. It's over."
Gabe Logan: "Don't. I mean it."
Singularity: "Your X34 gas darts, who do you think manufactures them? Who do you think supplies all of your arms? Look in the mirror, Logan. See your reflection. Look in the dark mirror!"
Gabe Logan: "I said don't!"
Teresa Lipan: "I heard. Gabe, I heard everything. I've got an agency mop up crew on the way."
Lian Xing: "She won't talk to me anymore."
Gabe Logan: "She's been through a lot."
Lian Xing: "She kept asking about her mother. I didn't know what to say."
Gabe Logan: "It's not your fault. She's just as bright as Addison was. You've been through a lot, kid. Let's head home."
Generally speaking, this level consists of first killing Singularity's guard detail, and then the Red Section leader himself.
Killing the normal enemies is more time consuming than difficult. You can hide behind the columns in the area where you start, or you can cross the first set of train tracks and hide behind the directories (posters?) if you want to get closer. Beware of the ones that cross the tracks to come for you; they're more dangerous to you if you hide in the middle behind the directories.
Generally the fastest way to defeat them is to snap to a column in the starting area, then peek out and make headshots when they peer out from behind the columns in their area.
They are all wearing flak jackets. They also all carry the SSP-90, if you want to collect refills from their dropped weapons. If you're lucky, you collected the C11 from the final boss at the end of the mission "Red Section". Another good choice is the BFG, available in the mission before this one ("Event Horizon").
The final boss (Singularity) is not very difficult, surprisingly. The technique for killing him is the same as for any enemy with the special armor save for the fact that he takes many bullets to kill. He is immune to your knife even when his armor is down. He will fall to the ground if you use an EDT dart on him. EMP grenades and your tazer will temporarily disable his armor but not knock him down - therefore the darts are the best way to go.
If you cross to the 2nd set of train tracks and snap to the ledge below him he can't see you. Eventually he'll get bored looking for you and stand mindlessly over to the right and look away to the right. He's easy pickings from this point. Climb up the ledge, fire an EDT dart on him and nail him with your gun with the highest damage when he goes down. Then just either hide behind the columns or go back down and snap under the ledge and wait, then repeat the process.
Once he gets bored and resumes his mindless position you can go back to pick up the C11, the BFG, fresh armor or more ammo and he won't see you.
For whatever weapon you choose you'll want to stand right on top of him to shoot him while he's down; the closer you are the more powerful your shots are.