Easy EXP and gil
When you get to the Ancient's Maze you should be able to find Greater Demons these beasts can summon Iron Claws to help them fight. Kill the Iron Claws when they appear do not kill the Greater Demon. Keep killing the Iron Claws. If all four of your people are alive you should get 1900 EXP for each Iron Claws you kill and when you decide to kill the Greater Demon you get 4000 gil and 2850 EXP.
More tricks, tips and cheats for this game are right here - Final Fantasy III cheats
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