Bockko, go offline and shoot up the drones!!
You can edit the offline settings so that you have lots of ammo and unlimited fuel. I also like to reduce the ammo effectiveness, so that it only does 0.2 times the damage that it would do in the MA. This means that you have to land 5 times as many hits to get the kill. (Once you can regularly get the kill in one pass with your chosen ride using these settings, you will find it a LOT easier to score kills in the MA).
Then just fly around the fields shooting at the drones from as many different angles and at as many different speeds that you can.
When I first started playing Aces High, I did this for about half an hour pretty much every day for a couple of months. Nowadays I will do it 2-3 times per week, usually just before I go online.
The other thing to keep in mind is that you should pick a plane and stick to it for a while when you are still learning to shoot accurately, as each plane has different characteristics, like the way they respond to roll while you are aiming, the view over the nose, etc etc.
When I first started playing my hit % was crap, but lately it is around 18-20%, which I believe is because of consistent practice.
Good luck, and be persistant, and you will get there!