To get the 3 million coin, attack one of the asteroids that don't shoot at you (It looks like a regular one without the orange dot). Its super tough, so use time stop or bombs to kill it, but it gives you a 3 million coin :D
Also, here's how to get the strongest weapon in the game:buy Infinite Bonus 1 (Black market) and 2 (Legendary Shop) and Rare Weapons (Legendary Shop). Then pick up the gold-looking gem to get Blue Lasers, and upgrade it with Double Fire.
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