First things first, I would highly suggest that for future game jams you use the ZIP export option from GameMaker, not only is it supported by default on Windows (didn’t even know RAR archives were still used) but it also doesn’t require any installation, which is a major deterrent for trying out the game. If you don’t want to use the ZIP archive then use the single executable export (I think all GM Studio versions support it).
Now onto the game itself. Really liked the graphics and the simplicity of the game. The instructions were also very nicely done. Unfortunately the music was quickly repetitive and the jumping sound effect didn’t really blend in the game very well. Other than that I think the game provides a nice base for a more complete future version. One small thing I’d change would be that the enemies die if they fall on the spikes (as I pushed a slime onto the spikes but couldn’t really kill it afterwards, so it just kept sending projectiles).