Once he makes it to the elevator shaft, wait until the three patrolling soldiers pass him and kill the sniper on the balcony before he spots Reed and alerts the rest of the guards. Don't use the sniper to kill him shoot the stack of crates or the AC behind him to distract them both.
However, if you use the nearby camera, you scan a safer path below, allowing Reed to move forward without killing any guards. Reed will remove the one behind the wall and give you a countdown when you need to kill the guard to the left. The next threat is the two guards sitting in the middle. Scan the power cables to identify the generator several floors above, then use the sniper to destroy it. The motion sensor after the first two guards needs to be disabled.
