itch.io

First, you need got a good boss in wave 3 (a boss spam every time 1 enemy left and after 1 enemy right) 

I think we got 3 or 4 boss like this. The best is the boss that never try to hit you, only turn around and place a mine. When you got a good boss you farm multiplier for got many cash

With cash, you buy many update and good weapons, and after you win some wave and try to find again a good boss. 

When you got many cash after this, you try to buy increase asteroid the more you can to be able to destroy more and get more point 

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