Wirbelwinds are best handled by two or more pilots working as a team. One draws fire, the other(s) will attack while the WW is occupied.
However, if you feel you must make a gun run on one alone, simply attack out of the sun (the sun should be directly behind you). They can't shoot what they don't see.
As for me, I just drop a bomb on them, releasing it from 2k above or higher...
I've lost only one aircraft to the WW since it was introduced, and that was my fault for not knowing he was hidden in the trees.
My regards,
Widewing