Methinks that to really take down the Russian collosus, it’s going to take teamwork between at least 2 if not all 3 Axis powers.
If Russia is reduced to building all infantry 2-3 turns after the initial invasion, the defending stack on Moscow will be insane (70-80-100? Inf), plus any FIGs the other Allied powers land on it will just add to the mass. It’s probably a safe bet that Germany can get a stack of its own in striking distance, with the right units to attack it (the right ratio of tanks and planes to infantry, just not enough stack overall to kill the Russian stack). Germany/Italy and Japan will probably have to have alternate stacks on Russia with enough force between them to chew the stack apart. I’m thinking a Japanese stack of like 20-30 ground units plus 10-15 planes, and a German/Italian stack with 40-50 ground units and 5-10 planes, against a Soviet beast of 70-80 inf plus whatever else is lying around, maybe 5 planes.
Japan would have to start off the initial attack from its stack in the Caucasus and essentially sacrifice its stack to get as many causalties as possible piled up (hopefully about even numbers for the units its killing off, 20-30 Russian inf or so), then Italy would follow (hopefully getting 5-10 more inf killed), and finally Germany would land the killing blow (probably just barely too, with the forces available~ 50 vs. 50 land units) before Russia’s turn where it would be able to build more inf again. This order could be changed so that Japan deals the finishing blow, but with the way things are set up in the Pacific with Japan unable to attack J1, I believe Japan will be arriving in Russia much later than Germany does (plus the Reds will have a turn to build some inf inbetween attacks).
The Allies would most certainly land more fighters into Russia once they realized what the Axis are doing, but this seems pretty much the only way Russia is going to be taken down (with a HELL of a lot of casaulties!).