In this stage, neither side has any planes nor the ability to build them. So it's back to the old school method of slow movement and counting hexes for your artillery.
The main stronghold of the enemy will be his northern depot, where he builds up a rather fearsome collection of artillery. Stay put, repair your stuff and wait for the opportunity to break through to the enemy factory south of your HQ. After that, you won't have any resource problems anymore and can build up the skills of your units while the enemy charges mindlessly. Your depot in the center will be under pressure from time to time. Heavy artillery, built in the south and transported by train, will do the trick. Once you have enough medium and heavy artillery trained to elite, assault his northern depot from two sides. It will be messy, but should work fine. You can also build up strength in the far south and try to rush his HQ, but this approach is not recommended as long as he has the train gun down there.