Thank you for the kind words! I know of this problem, but as I have decided on revolving the next update on enemy spawning, it will greatly be improved when it is out. Also, keep in mind that the slime will be the weakest and thus the enemy which gives the least amount of XP. As more, stronger enemies come, it will become easier to gain the requirements to level up.
This is what I meant by focusing on enemy spawns and enemy variations. This will entice players to keep playing until they encounter the stronger enemies. I would recommend experimenting with what types of slimes you could add before adding new enemies like goblins or the like. Larger ones, smaller ones, even changes to the color palette would be beneficial as I've seen you need help with sprites. This will be a good base for new enemies.