Could've just blockaded the planet and destroyed anything that tried to leave it or land on it. Also, just fling a bunch of asteroids at the shield. Not only is the ammo free, but it's also extremely plentiful. If the shield still doesn't drop, throw more (and larger) rockets at faster speeds.

And I don't know why people are thinking bigger = overpowered. Possibly true in pure-melee combat, but once ranged weapons come on the scene, being larger isn't really an advantage. If anything, it's a hindrance, because now your massive body is harder to conceal behind cover.

Also, giant ships are just missile spam bait. Or shell spam bait. I don't know why you're bothering to field them in the first place.