Well... i cleared it on "much harder" difficulty last week.
You need to upgrade your defenses early on so that you stand a chance against the waves of T1 units produced by the neutral factories. Get some regeneration defenses so that the rest of your defenses won't get torn down over time. Also make sure to make your way to one of the regions where the turinium genrators are. The neutral factories will send some T2 units there as well, so prepare for that. When you capture the 3rd turinium generator, the neutral factories will begin to spawn T1 and T2 units to help you. From now on the enemy will try to take out those neutral factories using air raids and/or dreadnoughts. The enemy will also try to strike your base directly with an air raid. If you manage to keep the neutral factories alive, they should push the enemy back by themselves (unless there's a dreadnought or heavy defenses). You just need to finish him off when you're at that stage of the game.