Before Trachtoum will divulge the secrets of defeating "Tidus", he wants you to prove your mettle by taking down a goobbue.
This quest is available if you have completed the quest Tales from the Tidus Slayer.
Starting the Quest
Talk to Trachtoum in Lower La Noscea to begin the quest.
Search for Goobbues West of the Grey Fleet
Exit the building and go southwest to the quest destination. Check on the purple glowing Destination and a Goobbue Farmer will appear. Kill it.
Speak with Trachtoum
Go back into the building and talk to Trachtoum to complete the quest.
Next Main Scenario Quest
After you complete the quest, talk to Trachtoum to accept the next quest: The Lominsan Way. There will be a cutscene.