I didn't have much trouble defending on regular difficulty and historical focuses. 10 INF+ENG+REC+SupART+SupAA. I had 50ish fully trained divisions with 20ish in queue. Plus maxed out mountaineers for the southern border. And I even't didn't own WtT yet, so no general trait bonuses or command abilities.
Standard "build CIV first, MIL later", no forts. The only things I researched was industry, infantry&electronics techs and superior firepower doctrine.
The single unusual thing I did was to convert all starting PZL.11 to PZL.24 with one factory in the beginning (it costs no resources, little reason not to do this)
I can post screenshots and the save later, but keep in mind it was a quick run without much strategising. I am certain it's possible to do this much more efficiently. You'd have a slowflake's chance in hell if you were against a human Germany though.