really simple, in every age you have ot build a turret and don't attack too much so that the enemys can be reached by the turret(s), in every new age sell the turret and buy a better one, in the final age build 4 turrets then no enemy can come through your defense, collect money and then build a tank and 3-4 super soldiers and click the special, the tank will kill the last enemys until it dies because of the turrets of your opponent, but then the super soldiers are standing in front of your opponents house and BAM
i don't even buy turrets until the 4th age(maybe 1 on the 3rd) and i beat. Just use your special for money, buy a tank unit and put 2 ranged units behind it. 5th stage just buy 4 ion cannons save up a buy a tank,tank, and then super soldier.