With anything you’ve got.
If you’ve built air, great, use it.
If you’ve got some artillery, use them here and there too.
The point is, you need to fight 1, or 2, or 3, small battles each turn, to stem the tide. A full retreat back to moscow, with NO RESISTANCE anywhere, is the worst thing the Russians could possibly be doing.
Surely the problem here though is that you presuppose the German player is using mini-stacks as opposed to two or three mega stacks as well…
To be honest I think if Germany goes full Barbarossa from G2 onwards then Russia has no chance, all they can do is buy time (and often not even that). And to be honest, I’m okay with that because if Russia could take Germany solo then it wouldn’t be a very balanced game…