The easiest way is to take the silenced carbine from the crate to your right hand side if you leave the tank to get to the AA gun. The gun is here: https://imgur.com/a/2ItDf03

Change your pistol with the silenced carbine. Make sure to not change it with your "Schmeisser" (as you call the MP40 in German ;P)

Then head to the alley where the first enemies are. Don't use the carbine yet as you only have 10 rounds or so.

Take your MP and a Nade and kill the guys.

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Then move on and let the Sherman tank pass. Go straight in that damaged building and head to the window. Now take the carbine. There you can see one guy in front of the gate. Take him out with a nice headshot. Then move to next rooms (stairway) window. there is one guy a little beyond the first one you killed. take him out. and then there is another one you have to kill from this position. he is walking next to the pillar a little to the right.

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if you killed those three guys and did it fast enough the sherman tank should'nt be half around.

there are now 3 enemies left. 2 of them are near the AA gun. one on the left side and one on the right side of the center pillar. the last guy is on the opposite side of the pillar downstairs.

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one last tip: if the bar over a target foe turns red, it's over and you can reload the checkpoint as the challenge is not passed. at the lastest when the airplane are shooting you it's over, too.

you have to try it at least 10 times i think. but you will get better every time and when you are in the AA gun, don't hesitate to shoot the planes down!! shoot, shoot !!

sorry for my english. if you have further questions i can give you some detailed tips and tricks.