I had found that if you tell your flak gunners to hold fire until the closing aircraft are quite near, they seem to land more shots.
"wait for it... wait for it... wait for it........ FIRE!"
There is a slight delay, but if you time it right, this method seems to dispense with the troublesome aircraft more quickly.
I've also noticed that the little figher aircraft still hang about after dropping their payload (small bomb). They don't even try to engage with their MGs... they just circle about getting shot at.