Hey, I played your game for a bit.
I'm on Mac with Chrome. The game looks and sounds fun however I had trouble with the controls for me "S" was down and "W" was up... Now on my Azerty keyboard, the S is above the W so it's really confusing to control the cube like that (D was right and A was left, which was ok, but I would much rather play either the arrows). But I played like this and managed to use the mouse pointer to shoot at enemies and pick up a couple of coins. Now when the "boss" arrives, the bigger blue cube on the right-hand side, I shoot at it as much as I can, avoiding its bullets, but I don't seem to hit it or damage it somehow. With the couple of games I played, I was able to buy better weapons, especially the one with 3 streaks of bullets (7500 coins), but that wasn't enough to beat the boss. When the boss is there, there is a small window at the bottom with letters going right to left, but I couldn't figure out what to do with them, I'm thinking there is a timing thing to kill the boss, but with no instruction in the game or on the page I couldn't find out what to do. I tried a couple of times to kill that boss but I gave up when I couldn't see whether I was doing the right thing or not... But congrats for your first game! You may want to include instructions and maybe some sort of back story to make it even more interesting.
By the way, if you care to check out my game Bouncenoid, https://jerkyhips.itch.io/bouncenoid I'd love to have some feedback too ;-)