Fun game. Found it very hard at the beginning, but managed to progress, get some upgrades and beat the boss. The boss was quite tanky and the fight took some time. I played again to explore some of the other shop upgrades. While farming I noticed that you can scam the shop owner by spam-clicking the upgrade during the spin rotation and you will acquire that upgrade even if you don't have enough currency. It will just give you negative currency value:
So I bought lots of damage and shot speed upgrade that way.
That tanky boss was not so tanky this time =D
Very good stuff. 10/10 gameplay and good approach to the limitation. The slimes and fire sprites are very neat. The whole art is indeed nicely made. Well done.