To get this quick you can select the Pavel mission (even if you aren't there yet) and use checkpoint reloading to get it done.
You must first escape from the rope and take out Pavel. Next you will want to walk forward and you will find a box with some gadget inside that you will want to pick up. From there turn around and head towards the cell in the back. On the wall next to the cell is another box. Press
to activate the gadget you just picked up and it will turn on some power. Head forward out of that room towards the locked gate. On the left hand side is another box you will need to activate power on. This will open the gate.
Now just head forward until you get to the door. Open the door and on the table directly in front of you is a knife. Press
to pick it up. You have probably already heard the guard whistling at this point and he is walking slowly towards you. Just come out from the corner with the knife and click
to perform a stealth melee. Now you can just press start and reload the checkpoint. You will start right before you grab the knife.
Here is a video showing everything from the main menu and including a checkpoint reload to show it all works.