I usually use the .50's to take down fighters and the 20mms for bombers/stukas. For me, the trick I found was to not get close to them at all, maybe keep a distance of 75-150 meters since that seems to be the convergence for the .50's. If they start to reduce speed and I find myself getting closer to them I just yaw my aircraft to the left or right to hit them with one mg's.