I got the beholder to under 800 hp but had to stop because it would have taken way too much time. However, I really like the concept, I think that's a really cool idea! The graphics are really nice too, I like the simple dungeon vibe.
The chest spawning chance is probably too low, I could often go 10 floors without seeing even one, so those particular runs didn't do much damage but took a lot of time. Since the floor are already growing bigger the deeper you go, I think you could reasonably spawn one chest per floor at a random position but perhaps reducing their healing value so the player would have to decide if it's worth getting or if the stairs are more efficient.
But I thought that was such a cool idea that could easily expanded with obstacles, moving enemies, and more. Great work!