From: Hoorn (NED).
An DoW on Russia defending this way should be made in a snow impulse in J/F or M/A. Clear up the speed bumps in Poland and the Baltics (and perhaps Odessa to) in that turn (If Leningrad isn't properly defended, attack it with Finnish troops. However beware: if you commit/lose to many units there, a couple of Soviet CAV might go racing towards Helsinki...). Than slowly but orderly move towards the Soviet lines. Make one front you're attack point (double stacks with ARM/MECH there. In the first fine weather impulse, use you're Offensive chit for a super combined, ground strike a lot of units on that front (all adjacent hexes), do a couple of high odds attacks on that front (the south is usually better than the north) to make breakthroughs. Next impulse, move you're units through those gaps and try to put the Russian front (with all those disrupted units) out of supply (no reorganisation of them). At that impulse, try to attack the cities (when you've got good odds, if not, simply let the Russian there intact. They will die later when you've got more hexes to attack them from...)
Keep some of you're airforce in reserve (or reorganise them with HQ's) so when the Russian armoured reserve comes you're way, you will be able to ground strike them.
Also: don't get disrupted early in a turn. If the turn might end soon, this isn't that much of a problem, so I would kill Russians later in the turn at lower odds (but always keep in mind that you, as the Germans, should take only Div or Inf losses).
The Russian has a choice than: to react by putting his armoured reserve and HQ's into battle (and those units are the best he has, so if you can ground strike them, always do so) or withdraw.
I also won't worry about entering Russia with a armoured kill stack ahead of you're front in M/J or J/A turns. As long as they stay organised, the Russians haven't got the forces to kill them, without making large gaps in the line.
Same goes for you're defense. One unit a hex is good enough. The odd bomber with low tactical factors you've got in reserve for defense makes it pretty hard to kill you. And if the Russians do succeed in killing an INF it doesn't matter at that point. It might probably mean more Russian units disrupted (nice to kill later...).
It is offensive you should focus on, not defence. Defence is only needed to prevent the Russians from putting you out of supply or recapturing cities or resources.