SMALL SPOILER WARNINGS
Requirements: Access to T. Gran Zedora Kingdom.
This quest begins at D-8 in T. Gran Zedora Kingdom. You'll know you're at the right area when you descend into a speak-easy/bar.
Talk to "Agatha" at the sword bar to begin the quest. Afterwards, speak to "Gibson" at a nearby table many times. He tells you to go the the T. Mersandi Granary Belt and deal 300 damage in a single blow to the mini-demons there.