Do you need gil?
Do you want to level up?
Well the best way to do both at the same time is getting a Large chain link
the best place to do this is in the ogri and nam yensa sand seas because you kill the yensans (people) and they will give you about 30 gil maybe more but if you link a large number they drop more items i did this and got a link of 70 the first time and i got over 100 Earthstones(it only goes to 99 but i had more however i only was able to sell 99), 20 earth Crystals, and more items that they dropped. After about 60 I got a Large link bonus which meant 4 times more items some times.
When i was finished i had 99 Earth Stones about 20 earth Crystal About 1000 Gil collected from the people and a few potions, hipotions, and about 10-12 Phoneix Downs

BUT BE WARNED IF THERE AVOID ALKAURANES AND FIRE ELEMENTS THEY WILL RUIN YOUR CHAIN AND MAY EVEN KILL EVERYONE IN YOUR PARTY