Killed a Panzer just now in a P38, in company with a 109. I got the kill, though I don't belive the 109 got an assist.
1) come in flat, you're much less likely to auger.
2) Yes, the dirt kicks up, but you can still see the white hit "sparkles" through it.
3) in the P38, at least, start firing a bit long, say 800-600 out, and as soon as you see the sparkles, add the cannon.
Took me about 4 passes, 400 MG rounds and 50 cannon rounds. About half the passes I zoomed in to fire, and zoomed out while firing (button on stick).
I think this method may be a bit hazardous for a flack, the flat approach would give him too much aim time.