Before fighting any boss (except Dracula), make sure to have the Hour Glass in your inventory. After defeating the boss, do not take Stage Clear Orb, instead wait for the stage clear song to play and use the Hour Glass (Jump and press Down) a split second before the Stage Clear theme finishes. If done correctly, you will start the next stage with all the enemies permanently frozen for easy kills! You can progress until the last level, but to undo the glitch (a must before fighting Dracula), simply use the Hour Glass again.
Beat the game to start again from the first stage. Score, level counter, and remaining lives carry over, and, insubstantially, enemy damage and speed increase. Beating the second loop resets the game to the logo screen directly after the credits.