Requirement: Received the bird Onlin from Mave and entered the Forest at night , reached level 80
Enter the Moon Forest at night and make your way to the Lemming X again. Before you talk to him make sure to equip all the Ultimate Saves you have or alternatively All Save. You will only have to worry about physical attacks so equip Adamant and Orichalcum to two of your characters. You should also be able to spare some Revival Potions and Goddess Scent.
When you’re ready then talk to Lemming X. You will need two characters to heal/revive most of the time every round. The standard team (Main, Grieg, Milka, Oreana) works but you could also change Oreana for someone with more HP like Lauren/Hero or even Elysha (in additon to her healing abilities she has a strong physical attack when equiped with her moon ore weapon).