One thing that has bothered me about the 'alien invaders' plot is that, well they completely lack any kind of military strategy, namely siege warfare, I guess that part wasn't written down in the 'aliens guide to invading other planets' manual. They are above the skies in orbit so they can simply blow up bases from that height. What needs to be established on is too why they are on the ground fighting against the human forces/resistance rather than going "Oh yeah we have space ships, hit the big red button!".
Couple of suggestions (feel free to add or disagree).
The first is somehow we develop a piece of technology that neutralizes their orbital weapons meaning we force them into a ground war, making them fight on our terms (If the enemy has range, you make sure they can't use that).
The other is that of desperation from the Alien invaders. Maybe their planet or star is about to go Krypton style and so they poured they're last resources into getting them off the planet and ours was the closest one near by to colonize. They maybe technologically advanced but we wouldn't be fighting an army, just colonists fleeing destruction.