A bit too difficult
Gameplay was nice, and the sounds were good too. What troubled me was that there were so many different characters to kill it was hard to kill all of them. It would be easier if one enemy is added after a number of days/rounds. It would also be easier if you could move around while using your power-ups, because it's so hard to aim the flamethrower when the enemies are moving around and you're not.
Because of the small amount of money, you can't really afford to buy that much power-ups. Maybe just put a price on upgrading the stock that you already have instead of keep on buying items that keeps on being wasted? For instance, you have 3 fire, 2 ice, and 2 stone. You pay the amount to upgrade the stock and that will be your amount every time you battle.
Overall, this game would need some improvement just to make it challenging and enjoyable.