After getting on the AA Gun in Mission 11: The Rhine, you have to shoot down attack planes. There are 25 total planes. They appear in the following order: 3 Middle - 5 Left - 4 Right - 4 Middle - 4 Middle - 3 Middle - 2 Left. If you miss a plane, immediately restart from the checkpoint. It is recommended to create a manual backup save before getting on the AA gun. Otherwise, if you miss a plane you have to replay the entire mission. When you reach the AA gun, pause the game and select "Leave" to go back to the main menu. Then, go to "PS4 Settings" - "Application Saved Data Management" and back it up using PS+ Cloud or USB. If you do not get the trophy, you can simply copy back the save game to start back at the AA gun. This way you do not have to replay ten minutes of the mission every time.