To be able to get this, first you have to find O'aka. Go to Macalania Woods and to his shop, that will trigger a mission to find him. Chase him in the woods until you eventually find him at a dead end. Then offer to hide him on your airship. This is available in chapter 1, but if missed can be done later.
Once he's on the airship, head to the cabin and buy stuff from him. He's highly overpriced but each purchase goes towards his debt. If you run out of space for more items, just sell some at the Gullstore run by Barkeep. You can earn a good amount of extra gil by using the thief's Pilfer Gil ability on each foe in battle. Truly, it shouldn't be too hard because you really don't need a lot of gil in this game and can use almost all of your earnings towards his debt. It also doesn't have to be done all at once.
You want to try and do this by the end of chapter 3. The trophy should still be able to unlock if you don't, but doing it by the end of chapter 3 is required for 100% completion.
In chapter 2 there is an easy way to earn 90,000 gil. Go to Guadosalam before you get three uniforms. Save before starting this. Then go to the inn and ask the guy behind the desk "got any data?" Buy the data. If the hint for who to sell to doesn't say "It's the last person you'd expect, no question" then reload. Once it says that, press the square button and sell it back to the guy for 100,000 gil. To get 10,000 gil to buy this easily, do Tobli's quest in the Moonflow, you can make an easy profit of 10,000+ gil.