Can be done on the prologue, best done after your first playthrough once you've got the hang of what you're doing. Load up the prologue on Cadet difficulty and make your way through it as normal. Shoot the German target with a carefully aimed shot and then make your way back to the broken stairway, wait until the two Germans below have stopped and then shoot them in the head being careful not to miss.
Go through the gap in the wall and then make your way to the door. Pause here out of sight and watch the truck stop, to the right of this truck will be a patrolling officer, wait until he turns around and then sneak to the left side. Pass around the mound of rubble and up the stairs, again being stealthy.
At the top will be a guard, he will notice something and come from the doorway at the top of the stairs to your right. Either aim using L2 or if you prefer use the scope and shoot, try not to miss or you will have to restart. Once he's taken care off just run for the exit marker.