Well really, if you give us the full scope of everything going on, we could give you a great way to do it on your own, we're not offering to beat this part for you, just an opportunity to view the entire battlefield first hand.
Missiles, imo, are a bad idea. A last resort at best.
Example, in my current game I have a key defensive location that absolutely, under no circumstances, can fall. In preparation for it falling, I have a nuke strategically placed to buy me more time. If it comes to that, I know I'm already in a horrible spot however.
Missiles on the offensive makes the AI mad, and often doesn't achieve the effect you want, and really, can often make your entire situation worst, or lose the game for you outright right there. None of these being a good solution.
Let us take a look, we might have a different perspective that could give you a good hand.