This game really brings me back to the golden era of like internet flash games in a great way. The core loop is interesting without being overcomplicated, the finale fight is tricky without being too punishing, and the game on the whole rewards cautious play and proper risk/reward evaluation.
I think you struck a good balance between dealing with the slimes being pretty each in a direct way, but the challenge coming from the management of your supplies - you're punished for missing, rewarded for strategically taking your time and positioning well, but (mostly) given enough room to bail out of most situations. I saw some others mentioned there were some spots it was easy to get overwhelmed, and I definitely see where that's the case, but I do also think the reverse is also a little true - down and to the right of the final boss, where there's an extra life machine and a slime faucet, I think is actually too strong of a position, perhaps. Even mid-wave, that was always a very safe spot for me to retreat to, where I could refill and then exploit the chokepoint of the entrance to easily top off on the larger slimes and push my way back out.
I think a couple of things could be a little bit clearer - I didn't immediately realize how much there was to explore, or that I could step into the slimey green areas without taking damage. But I think most importantly, it's very difficult to see how much is left in your slime tank when you're filling up the final can, which makes it easy to commit too much and end up without any ammo to continue. Making sure that your ammo bar is always visible even when pouring into cans would make a big difference there in particular.
All in all, great job! I didn't get the chance to try multiplayer, but I imagine that adds a lot to the game