Instructions
Go to Mushroom Rock Head in Chapter 1, and speak to Clasko there. Allow him to board your airship. At the end of Chapter 2, Clasko will throw a tantrum and leave your ship, running away into the wilderness.
Find the beacon in Calm Lands in Chapter 3. This is where Clasko ran off to at the end of Chapter 2. Speak to him again, and accept the "Clean Sweep" mission. Destroy all of the fiends in the ruins.
Return to Clasko at his ranch and let him know the ruins are clear. He will award you the Alchemist Dress and Sphere Grid.