Yo I also made a game about a slime, what a coincidence
I liked how you made some powerups interesting, such as multiplying and shooting more bullets at the same time. After enough upgrades, it's funny to see those monsters basically melting. Destroying enemy projectiles by firing projectiles was also pretty sick.
I'd suggest adding more sound effects, especially the dash sound effect, it would make juicier to walk around the map. Also unfortunately when I killed a boss the key got stuck in a wall, so I had to quit the game after getting 4/5 keys. Despite that it was a nice experience :)