Enter "42FFF" as a password to start at the last level (only for the New Hope Alliance).
When you start a level, make three or four Air Commands and Barracks or Manufacturing Facility (just to have some ground units in order to win). Deploy three or four (depending on how many Air Commands made) Heavy Bombers and after that, some ground units. After the Bombers reach the enemy's base they will cause a large amount of damage or destroy it. You may have to queue more Bombers if the enemy has SAM Sites, but it often makes up to three, so four Bombers would be enough. Note: You must have unlocked Aluminum and Silicon in order to make Air Units.