Easily kill any enemy
Equip the Cat Nip accessory on any character using the Gunner class. When their HP is 50% or lower, use the Trigger Happy ability. You will hit at 9999 on every bullet fired during Trigger Happy. This can kill every enemy in the game except two.
Earn Gil quickly and easily
After you give O'aka 100,000 Gil to pay off his debt, he will sell you items at very low prices. Hi-Potions sell normally sell for 50 Gil, but can be sold back to O'aka at normal price. Purchase 99 of them, then sell them back to get ~15,000 Gil each time. You can repeat this as many times as you want.