Re: Airships
Bah! Who cares about the ground pounders anyway* ;D?
They could always cower in buildings until the enormous death-dealing ship passes.
Airships would run out of fuel eventually and have to come down (you could make them hybrids so they derive lift from a combination of thrust and gas).
I agree that transports and airships which are important to objectives should stay in range, and the combat ships will have to come down to attack anyway.
You could always have the capital classes carry very little ammo and have to re-arm frequently, or be followed by ammo-ships (not sure this would be practical, but it would provide a weak link in an otherwise indestructable airship).
Another option would be to have the airships guns difficult to aim at targets on the ground due to elevation unless at an extreme distance (they are designed to destroy citys after all)... that would give the troops on the ground a chance since the ship would probably miss, it'd also encourage pilots to save ammo for important targets (like tanks).
Furthermore; I wish to dismiss the preposterous notion that a machine heavier than air could fly under its own power.