To do this you need to have a Racoon Suit. Once you have the Racoon Suit, go near the end of the first room you appear in. At the end of the room, you should see a door and a Dry Bones. Quickly take out the Dry Bones and go near the door and run so you can fly until you can no longer go up. While you're in the air, keep going right until you end up in a room with a small chest holding a Warp Whistle. Once you have the Warp Whistle you will not have to fight Boom, Boom. (the boss of the fortress)