I modified this a little and hopefully it will work better. I think mine is the only mission for ndisles2. When setting up a custom arena, it kicks you offline initially, but go back into the game and your arena will be there.
1. Create a custom arena ndisles2 terrain, default arena settings
2. Go to staged missions, select GNDTOAIa
3. Check load mission settings
4. Load mission
5. Click on populate mission.
6. Assign your self to one of the Axis planes (highlight your name, click axis double click one of the 2 planes, make sure your name is on one of the planes), click close
7. Click on start mission
8. Click on start now (May come up with 2 time boxes with a 15 in one of them. Change the 15 to a 1 and hit enter)
9. When the mission comes up, bail and end sortie
10. Click on kill mission and yes to the are you sure question
11. Move to field A40 in sector 9,7
12. Select a Wirbel, Osti, M16 or field gun and spawn in the hanger
13. Go to staged missions, select GNDTOAIa
14. Load Mission
15. Start Mission (do not click on populate) (May come up with 2 time boxes with a 15 in one of them. Change the 15 to a 1 and hit enter)
16. Start now
17. 5 or 6 P51's will dive on the field and 3 sets of bombers will fly over at 5k, try to shoot them. You can position your self so they are diving at you or passing to the side. Select one of the enemy planes with shift tab and the lead computing cross hair comes up.
18. Click on kill mission after the enemy planes have passed.
19. Repeat steps 13 to 18. Note that you have 10 time the normal ammo load. Steps 2 through 10 are just to get you in the right country which is Knights for the mission arena settings.