An F2 has only got the peashooter 15mm gun. No way that it is going to take down even a single Il-2. Granted they don't hit you in the fuel tank (behind your back) or in the oil radiator, under your butt
Maybe you should try to replace the 109 F2 with the G6, with mk-108, on the same AI level!! Real life reports state that sometimes even the 20mm shells will bouce off the skin of an il-2.