Practice practice and more practice.
As Lusche pointed out shots from the rear or above are best.
Make Each individual shot aimed, and make it count. Don't hold down the trigger, correct and aim each shot when the sight picture is right. At first it may take several passes to do some damage, but stick with it. If you can get a couple of shots off on each pass, in 3, or 4 passes your going to end up hitting him a couple of times and do some damage.
A pair of wirbles working together is tougher to deal with, so I like to save my bombs and rockets for them.
There are a lot of guys who are a lot better in the iL2 than I am. However I am enjoying learning this plane, and like anything else, some days are better than others. Midwar is my favorite hunting grounds, fewer fast late war fighters to worry about. And ussually there is someone looking to sneak a base or 2 if I keep my eyes open.