From your response to Bucket Mouse, it seems like you may not be aware you can get where they got without falling there.
Edit: Or re-reading and re-playing (there doesn't seem to be a way to fall there from above), maybe not, but I'd still recommend something different than just the enemy reload on that platform to discourage "stuckness" at this area. It was easy enough to get there that I also thought it was the intended route.
Edit2: I cheated and I guess it doesn't need changes.
I am mystified, however. Edit3: And now I did not cheat and am no longer mystified. I'm not sure how I wasn't able to do it, because I was trying to do it.
Boss exploit:
Enter room, leave room, come back. Boss is free.
Projectile Wrapping:
Which is a general NES Maker bug, I suppose.
There are state machine issues too. The worst one is you can't move left or right while airborne and holding B (due to the dash). Even when grounded, I'd actually rather walk while holding the button. (Suppose the user lands holding left+B. They should walk on landing. The dash could still be activated on specifically a press.) Let me know if you want other state machine feedback like this.
Edit: Actually one more state machine thing since it's worse than the others. If you punch, then switch to boomerang while punching the character is locked as if they were in a cutscene.
You get unlocked by pressing B or jumping with a direction, so it's not game breaking but still something worth looking into.
Getting switched to melee when there's no boomerang ammo was a bit annoying. I recommend instead that you simply make a punch happen when boomerang ammo is zero. The difference being that when ammo comes back, you will throw a boomerang without having to press select to get back to the boomerang.
Edit: Making the weapon switch sound effect to let the user know it happened could also help.
You can do the dash before you pick up the wind dash item in the beginning.
I'll agree with the others that I don't like the squeaks. It doesn't help that it's such a short loop.
Edit2: One more bug. This actually happened to me a lot until I tried to reproduce it.
Notice the huge warp. (Emulation was slowed down for this GIF).
Don't let all the nitpicks fool you, I think it's a solid start. Good job!