To kill a flak with guns:
Gun size or lethality isn't important. Volume of fire and good ballistics are; they help you putting a lot of rounds in that turret at reasonable distances.
Try to utilize the help of a wingman - one distracts the Ostwind, the other one closes in for the kill.
If you are alone, get some alt & seperation first. Observe your foe. Is he waiting for you or driving? If driving, preferably wait 'till he has to navigate through broken ground or obstacles, he might have to jump into the driver position for that. Have patience. Maybe he runs into an obstacle, if you see him trying to back up, attack immediately. If he's sitting at your town, wait and circle around him at 2k distance. When he's tired of waiting for you and starts killing buildings again, you strike!
Dive as steep as possible. That will hamper his aim massively while making it easier for you to put bullets into the open top. Get between him and the sun first. You may also not directly towards him but onto some imagined point 100yds to the side, then putting your nose at him at the last possible moment.
For killing Tanks with Hurri IID and Il-2 guns:
http://forums.hitechcreations.com/forums/showthread.php?s=&threadid=209030Hunting tanks with B-25H's 75mm main gun follows basically the same guidelines, but do not dive that steep and watch your speed - very easy to auger or overstress your GV.
(I think I will put up a revised article on tankbusting this week)