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
