To get to fight the Money-Money Boss in Terrace Cave, you must first get the Money enemies to use the "steal and heal" skill on your party. To do that, you must beforehand make sure to talk to Scatterbrained Khudol in the Casino gambler's hideout in Gohtza Low Town (he's facing the right wall), he will tell you about the boss of the Moneys. Then go to The Great Ancient Ruins and run around until you encounter a party of Moneys which are found in Astral Square. Injure them enough so that they use "steal and heal" on you, be sure to kill them only after that.