Play the game in any mode, and try to save as much money as possible during the first 
four ages. Then in the fifth age (future) get all the turret spots and fill them with 
Ionrays (the blue one). Almost as soon as the enemy creates units, they are destroyed 
by the ionray! Also, after they are destroyed, any stray blue circle thingies (NO idea
what they are called :p) will go and hit the enemy base! 
After about half an hour (on easy, it may take longer on other modes)you will win! 
It's costly, but it's easy!