If you're doing badly and your plane is heavily shot, land in a field & keep the engines running. Drive to a friendly airbase avoiding any trenches. Enemy planes try to shoot you but always miss, they do however fly in front of you so you can zap them.
After take-off, hit 'A' to accelerate time. You will now be immune to enemy gun fire. Now climb well above the clouds, until a certain altitude is reached. Soon, all the enemy aircraft will hang motionless in the air, allowing you to shoot them all down.