As the Soviets in the skirmish level "Island Hoppers", put one or two Tesla Coils at the sides of the entrances of the islands. There are three entrances, but only one is generally used. If you get a wall up between these Tesla Coils, then it will be very difficult for the enemy to penetrate your defenses. Also, building up a strong air force is good. Launching constant air raids is a quick and easy way to pummel your opponent into submission.