This one is easily missed if you're going for the correct ending. You basically have to kill Gebel, again if you've already done the Just a Flicker achievement, except this time you kill him with Zangetsu's sword, Zangetsuto. The problem is, that if you slash the moon when it turns red while holding the sword, you can potentially miss this. To avoid that, there's an item in the Hall of Termination called the Gauge Glasses. Wearing these will allow you to see an enemies health. Thus, slap em on and beat Gebel up until he's damn near dead, put on Zangetsuto, and slay him. Quite complicated, but nothing too special.