I came up with a staged mission to let you practice Ground to Air gunnery. It has to be done in a custom arena rather than in offline mode and the instructions look complicated at first, but it works kinda well. I may add some more planes etc so let me know if you have any suggestions. The instructions are below and in the attached pdf.
1. Create a custom arena ndisles terrain
2. Go to staged missions, select GNDTOAIR
3. Check load mission settings
4. Load mission
5. Click on populate mission.
6. Assign your self to one of the Axis planes
7. Click on start mission
8. Click on start now
9. When the mission comes up, bail and end sortie
10. Click on kill mission
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 GNDTOAIR
14. Load Mission
15. Start Mission (do not click on populate)
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.