Hunt Blue Spirits in CelestineCaverns to obtain Spirit Dusts. Then talk to Alchemist Isaac at the Celestine Caverns Entrance. Use your quest tracker to see your progress.
Alchemist Isaac: You're sick of it, right? I surely am! NakanoAzusa: Pardon? Alchemist Isaac: Vanquishing monsters in a tiny cave. It tires me just Alchemist Isaac: from watching you! NakanoAzusa: Well, sort of. Alchemist Isaac: I knew it! But you know - no pain, no gain. And the key Alchemist Isaac: is nearly complete! HiiragiKagami: I hope the key compensates for our work. Alchemist Isaac: Don't doubt that part! It's for certain that it will! We Alchemist Isaac: need but one more ingredient - spirit dust.