The mission progress was way too slow for me. I finished the 3 stage and than got the kill bose 1 mission. Like this, it's just stupid farming until I can try to just get highest score. I got level 33 and have bought everything but still coins left.
I would like to unlock all upgrades after beating the game.
A big problem for me is that killing a boss (or more at once) is puking so many coins that i have problems seeing myself and more important everything else.
But yes the game is really good and I had big fun playing it way too long
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