The second anti-aircraft cannon
After learning about the content of the shipments, you can take care of other elements of anti-aircraft defense. Go from the road to the small blue wooden building in the harbor. There is no one inside, however, you may find a few soldiers patrolling the area outside. If you still have the Panzerfaust you can shoot the anti-aircraft cannons (M4,10) from the distance while being in the building which helps you to avoid detection (M4,4).
But if you fail to do that, the only option is to stealthy eliminate all of the enemies by luring them into your traps. It would be harder to lure all of them and sneak to the cannon because one of the soldiers is an officer - they don't like to leave their positions.
The third floodlight
When you get rid of the cannon, go to the road. Follow it and go north to another building made of bricks (a possible checkpoint). From here you must get to the big white water tank (M4,5) that stands near the road further in the north. You can climb on it by using the yellow pipes. With a bit of luck you can run straight on it without being detected by any of the enemies.
Watch out - there is the second sniper on the crane (M4,6). You can eliminate him while you are on the road or after you get on the water tank - here, the shot is much easier.
From the tank, you must get on the roof of the neighbor building where the last floodlight is (M4,11). Destroying it completes the objective. To get on the next building and staying unnoticed, use a weapon with a silencer and shoot the truck in the south to distract the enemies.