Pretty enjoyable game with good visual clarity and easy to pick up, the monster designs look cute, and I liked the music.
I'd recommend keeping the camera centered, so you can see enemies coming more easily. I would usually camp in a corner and shoot bubbles from a distance because of this (I played the turtle the most, but tried each character). At some point the game would get pretty easy this way, with me picking damage + attack speed, and picking the enemy attack speed as penalty, so I would slowly become a bullet hell boss to the enemies. >:D
I found it harder to play as the slime than the other monsters, because of having limited range while my damage output seemed the same compared to the other characters (?), but I don't know if that's intentional.