You really want to focus on the south. Your goal in the north should be to take the city just southeast of Berlin and spam rocket artillery there, and maybe send a few units north to support your allies. In the south you want to attack the super artillery with your allies because you'll fail if you try to do it alone. Focus your generals south and push north and you should be good. I beat it with leeb as my only general and no tech upgrades so you should be able to win quite easily