This is a long one, again mostly because there are no planes involved.
You need to be very quick in the beginning. Cross the bridge in the south at once with all you can muster (sappers and one light artillery included) and take the depot there. Send one or two armored cars to your lone factory via the bridge and via the southern detour. Stay put at your HQ, the bunker and light artillery will do the trick.
After two close battles around the southern depot and the factory, you will be able to sweep the southeastern landmass and build a heavy artillery at the factory near the double bridge. Let the enemy come to you and build more heavy and medium artillery. Use one heavy artillery to clear some of the bunkers north of your HQ, so you can attack his heavy artillery easier. The rest is timing. Wait for the right moment to rush north from your HQ and over the bridge, ideally at the same time.
The moment is there when the AI starts moving its train gun into range, and ideally his heavy artillery at the same time. You should have plenty of experienced infantry and artillery by that time, and lots of transport cars to speed up your advance. A large and messy battle starts, but as long as you have enough experienced troops, you'll be fine.