Anywhoo, I found the final boss quite easy really. I beat it on my first go. That's what I love about this game; simple and sweet!
If so, errrm, just keep going in the direction where there are changes. I THINK so anyways, it's been so long, lol.
Oh, and the final boss is very, very hard unless you have loads of decent items, so I recommend going back, sell most of your badges and buy ultra shrooms and power ups galore!
Thanks for talking about this game, it's fantastic :)