In mission 9 you will be flying an attack plane and have to protect your allied bombers by shooting down enemy planes. In total there are two waves of enemy airplanes. The first wave has 8, the second wave 12. So there are 20 planes total and you must personally destroy 12 of them. Do this on the easiest difficulty and be as fast as possible, as your allies bring down the enemy planes quickly. Reload the checkpoint if you are close to starting the second wave but have a low number of kills.
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...