Fight bosses again and again
Believe it or not you can fight all the bosses again before you fight the end boss. You have to have EVERYTHING to do so. That means all the armour, weapons and stones. Just do the boss levels again and you will be able to fight lizard mans boss with bird man! and the cool thing is you turn into mouse man again and the same goes for all the other levels. To fight the end boss with a different character kill a boss eg. kill mouse mans boss with bird man so that you have pirana man. Next go to the desert door but dont go in in it. Next to the tower there is a secret door on the first or second ledge. Press up and you will transported straight to outside the bosses door! Then you can kill the end boss with a different character! Beware its quite impossible with lizard man but ok with all the others.