Thanks for playing and the feedback!
Putting the energy bar under the character sounds like a good idea. I was trying to find a sound effect for when you run out of stamina but I couldn't something that was fitting in time.
I didn't have much time to play test the difficulty at the end as I kept adding upgrades and nerfing the boss (too hard even for me for a while) without really testing how it all ended up so any feedback on the difficulty is very valuable. Do you remember what level you were when you faced the boss? I only faced the boss when I was around level 8-10 with lots of upgrades and items but it's probably too hard even then for a new player.