The game concept is nice! But I think the game could be enriched a bit more. For example, while waiting for the boss, the gameplay starts to feel repetitive. Maybe adding things like attack speed upgrades, damage boosts, or various abilities in between could help with that. Also, some visual improvements could go a long way. For instance, instead of a flat black background, a gradient could work nicely, or the orbs dropped by the slimes could have a small glow/floating animation.