where the girl drops a hacking tool for the elevator. She runs off to get 3 of her friends, so there's 4 enemies (and by the way they act they as well have thermal vision on, peeking seems to be pretty useless because they see me).
Anyway I have no idea how to pass all of them. The first time I got there I managed to draw one enemy into the previous room, get him to aim at me from the vent, went around and killed him. That proved useless the second time around when two enemies followed me there. Then I tried using the air purification system but they turn it off. The last thing I tried was hiding behind the pillar at the stairs they come down, then going upstairs and going around them, but after getting upstairs and turning on the air purification again, I ended up going head on into one of them.
After wasting an hour on this room, I'm seriously getting mad, what do I do here?