I played this mission for quite a while and couldn't find any help online for it. I finally figured out a way to crack it.
Don't waste your time with a conventional assault. On one attempt at this mission, I was able to clear the entire lower part of the board of all enemy forces and posted 4 Emperor Overlord tanks at the bridge leading into the enemy base. I couldn't force an entry and eventually hit-and-run rocket buggies combined with MIGS were too much for the force. I couldn't get a sustained air attack through the anti-air defenses and the enemy MIGS were a continual harassment that I couldn't seem to get a good counter for.
Here's what I finally did:
Build your base to the point that you can build a minimum of three Emperor Overlord Tanks, a Nuclear Missile, and a couple Helix's. Upgrade the Overlords and Helix's with Gatling Cannons. If you can place a single Emperor Overlord on your side of the bridge leading out of your base, it will successfully fend off any ground assault into the base. It's also a good way to quickly train your behemoth up to veteran status. When it reaches that point, rotate the next in. While you're building up and waiting for your forces or missile to get up to strength, use supporting arms (generals powers) to clear any anti-aircraft units from the bottom and right sides of the board up to where the civilian building war factory and tanks are south of the river at the east side of the board.
The basic concept is a well timed helicopter assault. When you have cleared the route, load two veteran emperor overlord tanks into the Helix's. It takes some practice, but it takes good timing to pull this off well. Launch your nuclear missile at the bottom right corner of the enemy base so that the fallout area will neutralize enemy forces and clear anti-aircraft for an LZ. With good timing, you want the Helix's to fly along the bottom, turn north, and fly up the eastern edge of the map to arrive in your cleared LZ just as the radiation disappears (for maximum effect). If you can insert your overlords, fight off the immediate counterattacks and then start working your way north and then west through the base. As you make progressive gains, ensure you're checking on your own base, making repairs, and keeping yourself from getting wiped out. It gets easier if you can wipe out the airfields - then your overlord at the bridge in your own base can fend off everything else.