Seems like I didn't even make it past first wave since I hadn't seen the transition and second enemy type. Actually thought that the game was endless before checking out the other comments.
Went for some more tries and made it to the actual boss fight but perished there quite quickly. Dunno if something is going on with my computer right now but every now and then (maybe each time garbage collection hits or something) the game starts lagging like crazy and becomes unplayable for several seconds. Does't really make it any easier.
Psh, can't give up yet, more tries incoming...
Finally! Can't say it was hard really, just tedious. The addition of enemies and boss even for sure made it more interesting but not more fun really. Quite the opposite. The slow slog to fight the boss enough times to beat it was just a slog. Just a test of patience. If you try to rush it at all, you'll get punished and have to face the boss with 1 or 2 hearts.
The more I played the more the lack of feedback started irritating me. Like few times I thought I was reaching the boss with full health just to realize that I had been hit few times without noticing. The flash on sprite is way too subtle and lack of sound doesn't help either. And the hitbox is too large too. Close calls are fun and should favour the player. Feeling like you didn't get hit when you did does not feel good.
The third enemy was boring copy/paste of the first one. No real difference at all. Well you couldn't be that close to them while killing and jumping over the projectile but wasn't such a smart strategy with the green ones either. The boss fight itself was pretty cool though even though the tentacles telegraphing felt a bit fast to my reflexes at least. And here too, the unruly controls didn't help at all. Kinda like in old Castlevanias where you had to plan your jumps beforehand but for completely different reasons. It's more like a turn based game than a platformer where you can play on reaction.