I found this level to be lengthy and challenging, but a couple of tricks won it on my first try. I ignored the central asteroid until the game was in the bag, and I finished before the battles got to more than a few hundred seedlings on a side. I'm big on the land grab and play a much faster game than you, Bonobo. Don't just sit around after you've met your opponents. Send out attacks as soon as you have the numbers to take one of their asteroids. 50-100 is enough if it isn't heavily defended.
They will tend to send single massive armies, which you can't defeat until they're weakened and the upper hand is already yours; let them go where they want to and retreat when they attack. When they move on, reclaim the asteroids they leave. Send multiple attacks on opposite ends of the front. The AI will not split its armies most of the time, and one attack will be crushed (retreat before you lose those guys), but you have a solid chance at getting the other, or at least weakening it for the next attack. Keep the enemy hordes on the move so they don't spend that transit time whooping your ass. Avoid engaging the main armies until after you've claimed most of the asteroids that fuel them. I don't know how most people play, but against single superior armies,an epic clash is the fast track to defeat. Distraction and relentless pushing at the weak points are the way to go.