I'm looking for a way to earn gil fast, since the items used to level up monsters get very expensive towards the end. What are some areas that have random encounters where the monsters drop significant (or significantly more than others) amounts of gil?
During end-game play, Yomi becomes relatively easy to farm (Archylte Steppes ???AF). The fight takes less than ten minutes. To reset Yomi you need to return to the Historia Crux and reload the zone. Then you need to switch the weather to anything other than stormy before changing it back to stormy.
Yomi yields a guaranteed
This related question - How Do I Defeat Yomi? - has information on strategies for defating Yomi.