I finished the game!
I have a idea, when you finish the game, it adds a playthrough to the save file, so the first time playing Quackventure would be playthrough one. Then when you finish playing playthrough one, it adds a playthrough. Then it will be playthrough two. The enemies and bosses will be harder.
This can go up to as many times as possible until it is extremely hard. If managed to beat playthrough nine, cheats are unlocked!