Finish 14 levels with laser maxed out to get the white stars.
Gather upgrade points from level 13 with laser maxed out.
Max out pulse and finish level 15, fly around in the middle and close to the boss, he will drop rockets. Avoid them as mentioned in one of the other comments. If he drops the white raindrops avoid them by going where no white drop indicator shows. If he drops the slow rain bullets, go hide right down in the screen. If he drops the fast rain, you just have to stay in an empty area on the right and move along. You will get hit, but you can take some bullets can't you!
Practice makes perfect.
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 "Civilizations Wars"){kind=icon}
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