All credit goes to Ryzo_Arashikage over on TrueTrophies for this one.
This achievement side quest doesn't start until disk 4
. You will need to deliver a total of 6 letters, all beginning with Kupo back in the Alexandria Steeple.
1. Deliver the letter from Kupo in the Alexandria Steeple to Atla in the Burmecia Vault.
2. Deliver the letter from Atla to Mogryo in the Black Mage Village.
3. Delivery the letter from Mogryo to Kumool in Ipsen's Castle.
4. Deliver the letter from Kumool to Mois in the entrance of Fossil Roo.
5. Deliver the letter from Mois to Noggy in Daguerreo.
6. Deliver the letter from Noggy to Kupo in the Alexandria Steeple.
Once you have finished the letter deliveries, speak to Ruby in her theater and you will obtain the bottle of Superslick.