The main objective is the easiest to complete and this is where you should start. Right after the mission starts, go left and walk over to the edge of the map, along the wall. Then, jump down onto the wall and cross the bridge (M7,2) to reach the islet - cemetery. Walk into the chapel and collect suppressed ammo for the rifle (20) (M7,3). You do that easily without putting up any fight.
While on the bridge, you can spot the boat - after you mark it, you will receive a secondary objective to destroy it. Also, it is a good idea to destroy it for the sake of extra experience and safety - it will keep patrolling waters along the water end.
Now, get into the building across the basement (M7,4) (go right past the bridge) and climb up. After you climb the stairs, go right and you will get into the building. Rothbauer's room is on the 1st floor (M7,5). In the room, there is a single guard and Rothbauer himself. Apart from that, watch out for the guard on the balcony in the main hall. After you assassinate the target, search the body.