Okay so where do I start... Well probably at the beginning ;-)
First off, I became addicted to the game because it was satisfying destroying the enemy. But it got boring after level 4 or so because the enemy where to slow and too easy to fight against (and you don't even have to fight against them... pls fix that).
2. Sometimes it was annoying that it was not really clear where the player hitbox is. Especially when collecting stuff.
3. You can't jump out of lava... I mean I know it was intended but if you take a closer look at video games like Mario, you can see that (for example) if you fall into lava you can't jump but you bounce off once. (maybe implement that)
4. At the beginning was to much to read... explain the player step by step
for example : Left Click here (arrow shows where to click).. See short text and easy to understand.
5. White text on bright blue background... Well I hope it's clear what went wrong here
6. I feel like the gravity is way to low (feels unresponsive)
7. This is more like an idea for your game : I feel like this game is made for speed running so please, please, please, follow that kind of direction.
The shooting is satisfying and "woshing" past enemy makes a lot of fun.
Maybe add a grappling hook or some unique weapon(or unique mechanics) and It will make a lot more fun.
(only my personal opinion!!)
I hope I could help you : )