tatertot
Whilst air to air is important, it's not always the easiest to improve upon.
Try these :
Bombing, take a B17/Lanc to an enemy strat, use the largest bombs possible and lay waste to it, return to base, reload on the arming pad and do it again and again, always return to base if possible for an increase in damage per deaths/sorties.
Use the largest bombs possible to increase the hit percentage and reload on the pad to increase damage per sorties, keep on bombing when possible to increase damage points.
Attack against Ground targets is the same method as above.
Attack against GV's, kill as many GV's as possible as fast as possible, land the kills again.
GV against ground targets, take HE shells, lay waste to a town, return to spawn to land, repeat over and over again.
One exception is take a PT boat to an enemy town, use rockets on the town, taking care to hit something and you will get a hit percentage in this category, rockets and torps only will work for this hit percentage.
GV against GV, drive to an enemy spawn point, set up correctly and kill anything that appears, again land the kills.
Air to air, this is the toughest to increase because it's the only one that requires any skill, the others just need time and targets.
Good luck
wipass