When you're in your panzer and an enemy Flak is snaking his way toward you (they do this to make themselves harder to hit once they know you're there), fire HE (high explosive) at his tracks, rather then AP. You don't have to hit the flack, as getting close will detrack him and keep him from getting close enough to kill you.
HE will also kill M3's, M16's, M8's, and the engines of Panzers and Flaks without actually hitting the vehicle. Just get close.
Once they are detracked, or motionless, shoot them in the side of the track area with AP and they CAN explode with a single hit. Not always, but this does work and highly demoralizes the enemy.