To get this you have to beat the Gorgon which is the final boss of the game. I didn't find the game or the Gorgon very hard so getting to the end is just a matter of playing the game. By the time you get to her you should be quite strong, hopefully close to level 20.
What I advise is to use your invincibility magic and attack her, once that wears down switch to your beast mode pending it's full, if not use your beast potion, and attack her. By then your magic should be close to full again, if not just use a blue potion and the invincibility spell again, and just repeat this formula.