Not all that hard. The computer uses his planes in stupid ways, so you can easily dispose off most of them early on. It will take a few turns before ground battle takes up pace, so you have plenty of time to arrange your units. One medium artillery each south to your factories, and one transport truck with one infantry unit east over the bridge to take the depots in the NE corner of the map. Take care of your planes and repair them soon enough.
First, use your resources to repair only. Once the initial assault is over, you will have resources enough to build more Gotha bombers, which are very powerful when experienced and at full strength. You will be strong enough to assault his factories near his HQ relatively soon. Do this to as soon as you can to cut the computer's resources, even if it means taking a few hits from his medium and heavy artillery.