Well execution of a simple mechanic, the music and sound are very fitting. The visuals are okay, I'm just a bit confused by the mix of top down and side view - your character is top down, the shooting clearly is from a top down perspective, but the slimes and the gate are drawn from a side perspective. To be fair, rendering the slimes sideways adds a lot of flair, and not rendering the character from top down seems to be odd in this setting, too, so I'm out of suggestions in this matter.
The first times I played the game, it happened quite quickly that it ended so suddenly without me being able to tell why. I was sure I hadn't missed any evil slimes or killed too many friendlies. It was quite frustrating and took me some time to figure out that touching any slime makes you lose health. I must have missed that when reading the description, but a simple feedback in-game could easily prevent this confusion. Say a hurt sound and/or a brief red overlay.
I had the feeling that the waves where varying heavily in length or slime count, and I mean not just increasing. Could it be that moving around causes more slimes to spawn? Didn't really investigate this.