You'll start in the Elevator room. Listen to Bain's speech: you have to destroy all of the cameras within 7 seconds, or the alarm will go off preventing you to beat Plan A.
Cameras: The cameras spawn in the Elevator room, Reception, South Corridor and North Corridor. Spawn positions are random, so before you draw your guns look for all the cameras. There are 9 cameras in multiplayer (with a full crew), and only 3 in singleplayer. You need a silenced weapon to destroy them, otherwise you can just melee, but be fast. Warning: don't go into the ICU area yet, the guards will see you with your guns and will sound the alarm.
Panic buttons & Civilians
There are 3 panic buttons in the area. Preventing civilians to press them is a must, as they will sound the alarm right away. Panic buttons are located in Elevator room, Reception and North Corridor. Many civilians will spawn in the area, so some of them will try to press the other panic buttons as well. The best thing to do is, before drawing your guns, organize yourself and with the crew: one of them should guard the panic button of North Corridor, another one guard Elevator room and the other 2 should stay in reception to do the objectives and keep the civilians in check.
Keep shouting at civilians! If you don't yell them for 10 seconds, they will stand up and try to press one of the panic buttons! Warning: don't kill a single civilian! You will raise the alarm!
In any second, a guard will come up to check if everything is OK. Bain will tell you when a guard is coming. Kill the guard with your silenced weapon. But keep an eye on the civilians, only one of your crew should take care of that guard. Sometimes, a repairman and civilians from the elevators will come too. The guy guarding the elevator will take care of them, as they are not that dangerous and will only try to press the panic button.
When it's time to search for the patient files, only one of the 2 guys guarding the reception must search for them, while the other one keeps shouting at civilians.
When you got the right file, it's time to put the sentries on, but still remember to keep the civilians in check again. If you haven't encountered a guard before the sentries, then it is not over yet. A guard will come up even after you put the sentries. Keep an eye on the civilians while one of you will take care of that guard.
Time for some scrubs
Wear doctor clothes. Talk to Doctor Eriksson in the ICU area and don't draw your guns yet. The patient you are searching for has blood poisoning, abnormal heart rate and he shouldn't be recovered soon. Listen carefully to the doctor. The doctor will talk twice if you want more info about the patients.
When you know where the right patient is, don't draw your guns yet but look for the alarm box in the Construction. You'll notice it when it is opening and one of your crew says "I'm near the alarm box". There are 3 cables, and you will only cut one of them but which one? Let's take a look at the image. The Isolation C has blue wire. That means if the right patient is inside that room, you'll need to cut the blue wire. If he is in Isolation A then it's the red wire, otherwise the yellow wire for Isolation B (You'll notice it in-game).
Now it's time to draw your guns, but DON'T shoot yet. The guy taking care of the wire must hold the F key (default) to cut the right wire, and if you shoot the alarm will go off and all of the isolation doors will close automatically. When it's all done, look inside the isolation for the patient, and do the rest of the job. You are free to shoot and kill now!
How to beat Plan A in Singleplayer Hard/Overkill difficulty
Start: make sure that girl dressed in white isn't in the elevator room. She is the one who triggers the alarm so if she's not there you don't need to take care of the elevator room at all. Otherwise, if you can, tie her.
Yell around: when you shot all the cameras before the countdown, yell at the civilians in reception first, make sure all of them are on the ground. Then run to the north corridor and yell at everyone. If you can, yell at the civilians in the cafeteria abit. Then return in reception and so on... Make the bots go around, they can help you in the first part.
First phone call: after the first telephone call, the bots automatically start going into reception and camping behind the desk all the time. So you don't need to take care of reception anymore, the only thing you need to do is staying at north corridor and yell at the cafeteria sometimes. If you want help, just yell at one of the bots and they will come (do NOT yell at one bot repeatedly. Every yell just freezes the bot by half a second, and that's what most players do all the time making them stand still and complaining on how stupid they are). Remember that when you start yelling at them, they will switch the gun into an Amcar.
Guard: wait for the guard to show up and melee him as soon as he get upstairs (unless you have a silenced gun so you can kill him faster). Do NOT let the bot come to you while doing that, it will shoot the guard with Amcar and set off the alarm.
Repairman and civilians: very simple, if they will appear yell at them, then go back to north corridor and yell, then to the other civilians and yell, and so on. With civilians coming from the elevator the stealth will become harder to do, especially when doing the patient files part.
Patient files: when it's time to search for the files, let the bot reach north corridor so he can take care of the civs while you are searching for the files. Remember, 1-2 files each, then go back to north corridor, yell and make the bot stay there.
Sentry guns: be careful, civilians can still trigger the alarm even if it's time to put fake sentry guns. Use your bots.
The rest is very simple, listen to the doctor and then cut the right wire.
When you succesfully beat Plan A, you will be rewarded with 2 achievements:
Don't Panic - Complete NO MERCY on hard or OVERKILL without any civilian raising the alarm. To complete this challenge, you will have to have played the heist from the start.
Or was it the blue one? - In NO MERCY, cut the correct wire opening the ICU door security system. To complete this challenge, you will have to have played the heist from the start.