To add to the above:
Aetius is right. You have to think one thing: brute force.
The most important fundamental thing to do as the Russians is to just sit there with your troops. This might sound strange, but just by sitting in place in the path of the Germans, you can accomplish several things:
1) You are not disrupting your own units (from opportunity fire) on your own turn. In a word, force the enemy to use his valuable MPs to disrupt you from fire, don't make it easier on him by disrupting your units on your turn. On the other hand, and this is adding to what Aetius said, once your units are disrupted and in a stack, then don't be afraid to fire them all off at the same time at the enemy. If they are mostly disrupted then it doesn't matter, just get some shots in anyway. So the trick is to avoid getting disrupted first, then once they are disrupted - go ahead and fire them off.
2) You force the Germans to assault in order to take ground.
And going more into detail, when you are "sitting there" try to do the following:
1) Try to create stacks of mixed units to solidify the defense, and especially avoid having one disrupted infantry unit in the line (stack several disrupted infantry units together).
2) Avoid blocking your large stacks in and denying them the ability to retreat by creating stacks behind the front line. This is actually very tricky to keep a handle on, you will have to sometimes resort to checking the right click status window to see how many troops you have stacked in a hex, and reference the surrounding hexes too. The worst thing you can do is have a nice fat stack on the front line get killed off because it cannot fall back when it gets assaulted.
All four of these points work to accomplish the larger goal of trading lives for space and time. By sitting in front of the German advance in large stacks, the Germans have to assault them and incur fatigue which in turn will gradually grind down their advance and force them to rest. Sure the Russians lose men and incur the same fatigue, but they can afford it if you manage your reinforcements properly. In the places where you are passive, use your artillery and tanks to do the attacking (by fire).
Having said that, the
most important tactical thing to do as the Russians is recognize where the main German effort is, then do localized counter attacks in the "other places" to force the Germans to have to diverge from the main effort, to backup / reinforce the "other places" that you are threatening and it keeps them off balance, unable to concentrate on their primary push. In other words, when playing as the Russians you MUST counterattack in order to actually defend anything, rather than doing the traditional "sit and be passive" all along the front which is what a typical defender might be used to doing. Having played the Prokh scenario many times, you cannot (as the Russians) be passive everywhere. So what you do is, you become passive like a sloth in the path of the German advance (using the "sit there like a rock" tactic mentioned above (aka. plan "tree sloth" ) and in the places where the Germans are not attacking, you should be active and causing all sorts of problems with localized assaults to try to attract attention. Once you attract attention, you execute plan "tree sloth" and go passive, then use only your artillery to fire back in those places, move your reserves to the now inactive areas and execute plan "attract attention" again. This is in a way what the Russians would do historically when you read text that the Russians launched constant counter attacks. I assure you that you can drive the Germans mad with this tactic and if the Germans do not counter your localized counter attacks then you just might break into their rear and take out some German support units. So, essentially you have to take the initiative away from the Germans by counterattack so as to put them in a lose lose proposition: either they pull some strength away from their main effort, or they lose ground in vulnerable places. If the Russian does not counterattack, then it affords the German player the luxury of *concentrating* where he wants, when he wants, and the Russians will inevitably lose if the Germans maintain complete control over the situation like this. So, ideally, as the Russians you are trying to force the Germans to spread out his strength to a degree that he is no longer concentrated.
Of course the other thing about the Russians is that your tank (and artillery) units should probably be the only thing that is actually firing on the enemy, just let the infantry units sit in place 99% of the time. Once your infantry are at a numeric superiority (in number of units in a stack) in an area, then you should go ahead and start firing with them. And speaking of artillery, Russian artillery is second to none. If you are not using it each and every turn then you are not being a good Russian. Don't underestimate the need to check for detached units too, because it might turn out that the artillery you keep calling in is at a reduced effectiveness. And in passing, don't forget to assault with the combined arms stack of SMG and T-34 units -- don't assault independently of each other, it is a waste, and where possible don't waste your SMG units holding terrain unless it is absolutely necessary. Use them instead as an assault force that stays 1 hex behind the lines waiting for the right time to strike with the tanks that are up front.
And finally, as the Russians, don't be afraid to assault the Germans even if the Germans are not all disrupted in a hex (as long as the Russians outnumber the Germans in number of units in the assault!). You can usually force a costly assault upon both sides by doing this, of which the Germans cannot afford to sustain. Then, where possible, bring up fresh units and continue that costly assault against the same unit -- something will have to give! It is better to do this though once all the Russian reinforcement divisions have arrived on the line, at which point you will outnumber the Germans by a wide margin and should have a nice depth to your forces.
I did things like this in the Prokh_Alt scenario, and in one result I actually ended up running the Germans all the way back to the Psel on the west side. However, it is certain that although both sides have a shot at winning, it is indeed very tough to do so (with both sides). Both sides really have to know the strengths and weaknesses of their units!
I hope my random rambling helps.
*edited for typos*