This is a missable trophy/achievement.
The second time you fight Zangetsu, he gives you his sword, Zangetsuto. You can then fight Gebel.
While fighting Gebel, if you invert the screen and chop the moon, you will miss this trophy/achievement and need another playthrough or back-up save to get it.
Instead, to earn this, simply beat Gebel using Zangetsuto without touching the moon. After this is unlocked, you can then reload your save from before the fight, invert and slash the moon, and proceed to the third and final ending.