First, defeat Captain Hook and seal the Keyhole in London. Return to the area where you were fighting Hook and wait for some Rare Mushrooms to appear. Go back two rooms so that they have a chance to appear. As soon as they are on the screen, target one of them and make sure that you are flying first. Hit it once so that you have it in mid-air. Then, keep hitting it while you are in the air. Make sure you do not let it fall or land on any part of the ship. Doing so will cause it to vanish. You can keep it in the air easily, as long as you are also in the air with it. This allows you to get easy Mystery Goos, Elixirs and Mega Exlixirs. Repeat this trick as many times as needed. Also, try to aim them away from the ship when doing this trick.