First, equip the Cat Nip accessory on any character that's using the Gunner class.  When the character equpped with the Cat Nip has her HP reduced to below half of its maximum, use the Trigger Happy ability.  You will be able to score 9999 damage on every pullet fired during Trigger Happy, which will kill every enemy in the game except two.

 

After giving O'aka 100,000 Gil to pay off his debt, he will sell you items for extremely cheap prices.  He sells Hi-Potions for 50 Gil, but you can still sell them for normal price.  Buy 99 Hi-Potion at 50 Gil, and then sell them all, you can make over 3 times the money back.  You can do this process over and over until you have the desired amount needed.