So kudos out of the way first, this is probably the most impressive use of the RPGmaker I've ever seen. Not only does it look superb, but the controls are nearly flawless. Most sidescroller/platformers made in rpgmaker feel very janky. I have no idea how you made this feel so smooth. Stellar work.
However, as others have mentioned, it's brutally difficult. Figuring out the key to damaging the boss in each phase take a bit to work out. I made it to the 3rd phase of the spider boss, but never figured out how to damage it. And it took a while each time to get back to that (if I even made it), so it eventually became frustrating enough that I decided to stop. I don't mind a challenge, but having to replay all the phases just to try to guess how to hurt the third one was a bit of burn out.
I think adding some way to restore health along the way would help a lot. Maybe just some healing item that pops up on screen from time to time and gives the player something else to jump at. Perhaps tone down some of the damage of the environmental hazards too (especially the purple bubbles, which wrecked me a lot). And give a few clearer clues to what hurts the boss (breaking the web string was a very good example of an intuitive damage mechanic).
That said, this game is so great that I'm sure I'll have the itch to go back and try some more (I'm already thinking about it). I'm completely amazed at what you've produced here, especially since you say you did it with just eventing!
Edit: finally got the first impression video of me playing this uploaded, should you want to see precisely how I struggled: https://youtu.be/s50C5GXeQl4?t=3287