With .50cal versus Japanese and German medium bombers you want to cluster your shots. The biggest mistake players can make is spraying their target without focusing their shots. It's somewhat ok with a large 30mm or 37mm cannon as even a spray of shots will cripple your target... with small calibre weapons you need to focus fire.
With a G4M or Ju88 I find the best place to aim is from above on the spot between the engine and the wing root. If you need to make more than one pass then keep focusing on the same area on either left or right side of the aircraft. I find this is a good spot because it is where the fuel tanks are and any hits to adjacent parts are either the cockpit housing or the engine.
I prefer an overhead pass with deflection angle. Never attack from directly behind (dead 6 approach... 'dead' being operative
) and always make sure you have extra speed so you aren't on target for too long.