It's a very nice game some things I, personally feel like could be improved would be:
Adding some sound effect variation so maybe the pitch changes a random amount each hit or maybe it plays a random sound effect from a few available.
More upgrades, as in maybe you should be able to unlock more than 8 upgrades ( Life steal is great but maybe it should be less powerful as I collected quite a few of them and managed to just keep clicking the middle of the screen and healed back up and progressed without doing much ).
A little bit of visual polish would be nice, like removing anti aliasing from the font when in game.
Otherwise, well done!