After getting on the AA Gun in Mission 11, you have to shoot down attack planes. There are 25 planes total. They come in waves in this order: 3 Middle - 5 Left - 4 Right - 4 Middle - 4 Middle - 3 Middle - 2 Left If you miss a plane, restart the checkpoint immediately.
Here is a video guide with English commentary!
Here is a playlist of guides for this game!
https://www.youtube.com/playlist?list=PL-HeW6XkKdUKe25WzwEqI...