Before we start, let me bash in your hopes: You cannot use with the sniper rifle you got from the previous level to shoot Hannelore while she's sitting on the balcony, finishing the level in under 30 seconds. In order to make this tactic impossible, the developers make it an objective to drag her body to a specific location first. Also it might be really hard to actually get that shot considering she'll immediately walk away once the level begins. The opportunity is there, just incredibly difficult.
Silent assassin rating
-Watch the guard right in front of you, he can do one of three things: he can remain still for a very long time, he can walk up the path to your left, or he can walk up to the boat you're hiding behind and wait there for a while. Both the first and third behaviour lend themselves really well for subduing, but for a proper Silent assassin rating we'll go with the second. This because the level will probably take longer than the sedation will last.
-Sneak past the guard and go to the sewer entrance. Don't enter it, instead use the wall as a cover from the helipad guard and wait for him to return. Once he's past you, sneak to the back door guarded by a simple guard and wait for him to go take a piss. Sneak past him into the hospital.
-If you are going with either of the other two behaviours, drag the sedated guard out of sight. Contrary to the claims of the walkthrough video, this is very much possible. You probably won't even have to move the body if he walked to your starting area, and the hill to your left will cover him from the helipad guard completely if he didn't move. You can now either take the same path as described before, or head through the sewers. In the sewers, there are two exits: the fountain (marked on the map) and a small ladder (not marked). You cannot enter the building without raising an alarm through the fountain without wearing a uniform (though alerted guards through a found body don't matter), the ladder might be a different case. (untested, though the last editor claims it to be possible)
-From the back entrance, take the first room to your right and run through the rooms until you find one with a cultist disguise. Put it on and continue on to the agency drop point. Be careful of the guard on his way to the bathroom, he's more observant than his colleagues and probably will see through you if you don't wait for him to get into the bathroom first.
-Take the key and optionally the poison, although I found the latter rather redundant in this level. There are also a Kalashnikov and a silenced Beretta here. Head the last point of interest on this floor to collect the patient disguise and then go to the stairs region. Note that you cannot get past the guards without raising an alarm in any other disguise.
-Try to walk by the nurse without getting seen, depending on when you reach this point this can be really easy or difficult. If seen, she will take you to Hannelore who mistakes you for a German patient of hers. While Hannelore is talking with her back turned to you, strangle her or poison the water. Before a patient comes up, drag her body to the hiding place. Ensure that the guards do not see you.
-While the method above dos work and allows you to get Silent assassin, it's much easier when you don't get seen by the nurse (at least at normal difficulty). Walk past the guards and into the room next to Hannelore's office. From here, you can sneak into her office and strangle her with the fibre wire. She is quite oblivious to everything that happens behind her and won't even be alarmed when you walk while opening the door. This way, you can choose the opportune time to kill Hannelore: you won't have to listen to her long story before she dies and you won't be pressured by the patient approaching or have the opportunity to wait for him to leave first.
Note: The patient mentioned will occassionally come up to Hannelore's office and walk to her desk before turning around and leaving again. Especially when listening to her story first, he can be an ill-timed problem. When she's not at her desk, the patient will start searching the second floor for her: First the room to the left of the office, then the one to the right, before going to the southern rooms. Here he'll freeze indefinately. If cornered, it is best to hide in the medicine closet in the room to the right. Not behind the closets, the patient will walk behind these in his search.
Drag Hannelore's body going straight through the center room, don't try using the cover to your left or right because the guards will see you. Stash her in the room you have to leave her and return to the exit point. With the patient disguise this shouldn't be much of a problem, but you can pick up either the cultist disguise you left or the guard uniform next to the pond if you'd like.