The proper way to get to door 40 from Floor 15, from a user on Gamefaqs.

[Ereson]

Taken from gamefaqs.com section for Umihara Kawase, from a user called: "Pebble_Raven ":

I figured out how to get to the boss field 40 which was exluded from the only walkthrough for this game. At the top of Field 15 when there's a wall on the left, beyond which is a door, and another door on the right, use the fishing pole on the far top left of the high wall, then extend the line as far as it goes. Walk to the right until you fall down a small bit and the line is at the end of its reach. Jump and hold down, and keep holding down until you're snapped to the top left after one bounce. Once you're at the wall's top, hold left and you'll climb up onto it. Then just drop down to the left and walk to the door.