Cool concept! I will say though that after my 3rd scale up I got stuck in a situation where no guns ended up spawning and the aliens slowly shot me to death.
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Thank you! Indeed 4th wave is a bit different, where you couldn't shoot aliens, this asteroids which you get is actually a shields, and you need to cover your planet with them.
You still can rotate planet with small amount of shields to consume bullets (basically when bullet hit asteroid with shield it deals no damage), so the idea is to collect enough asteroids to cover the surface and absorb them just after that, it will be enough to progress to a next level