I liked the idea of combining slimes to grow stronger, and I really liked the idea of being near a start point to heal. I do wish the tuning was more in the players favor - enemies attack very often and do a pretty significant amount of damage. The healing is slower than the enemy attacking, so once an enemy is on you, it feels almost impossible to escape. Even if you get back to the healing station, you are taking more damage than you can heal. That's how it seemed in my playthroughs, maybe I missed something.
I really enjoyed the choice of music and art. I think the music kept an actiony rhythm and vibe, and the art had a cute, lighthearted vibe.
I think you could benefit from camera shakes, hit stop, and particle effects. I don't know if you're engine already has a particle system. If it does, adding some particles will make a big difference. I am not sure how camera shake would be coded in your engine, but this is how I do it in unity: https://imgur.com/a/rWEMD0U
As for hit stop, this is how I do it in unity
IEnumerator hitStop(float hitStopTimeScale, float duration)
{
Time.timeScale = hitStopTimeScale;
yield return new WaitForSecondsRealtime(duration);
Time.timeScale = 1f;
}Please don't take my suggestions as me not liking the game. I liked the game in many aspects, I just think it could be even more fun and easier to pickup for players.