A good starategy is to build advanced barraks (save money). Then buy a power plant and then residential upgrade level 1. From there you can buy a tank factory or an airport. A good army is rocket soldiers, medics, a few choppers, and some tanks. I only lasted about 6 waves.
Heres a good idea: you have medics for infantary, why don't you make mechanics for tanks? I hate having to buy tanks over and over again. WARNING: DO NOT BUY AN ARMY THATS ALL TANKS.