I figured out how to get the 10 hits with the flour bombs in the flour bombing mission.  Set up a bombing run:  Get aligned with the target and hold down shift.  When you get into position, quickly hit D a couple of times, like 4 or 5 times.  If you did it right, you should be able to get at least 3 hits in one run, which makes it possible to get the 10 hits the mission calls for