Doing what the Russians did in 1942 will probably help you here as well...save your reinforcement cards, retreat back to Stalingrad, but try and hold in front. Use your 8 pt Fortification card to fortify in front of the city. In the Caucaus as well..retreat down to the river um...Terek and the mountain passes. Save your political points and buy the Tblisi supply card. Try to have at least a rifle division in every hex along the front.
Use the Don as much as possible...try and give your units at least two turns before he makes contact to 'dig' in. The longer the line stretches for him, the harder it will be to surround/push.
At an opportune moment, play your reinforcment cards on waiting HQ's (in cities on rail lines) near where you want to attack....then hit his lines. His air recon cant spot your units if they are not on the map.
Forget using the Russian airforce until winter...but keep your fighters well enough back so they can intercept, but not be attacked by his fighters directly.
My opponent believes in the two hex rule..if you are two hexes away, the Germans will not have enough AP's to launch a serious attack....or to exploit if they do.
“My logisticians are a humorless lot … they know if my campaign fails, they are the first ones I will slay.” – Alexander the Great