A lesser-known destination is the Castle of Ordeals. Look to the east of the islands in the North-Central region of the map. (Remember B + select is the world map).

You can get some good experience from the castle. I would go Castle, Ice cave, Desert (to get the airship). Then you can go to the Islands and get class change, to Gaia, and to Onrac (which leads to the Waterfall and if you go to the Sea Shrine, Lefien). Then you can go to the Mirage Tower and get Adamant, giving you excalibur. Then, you can return to the Gulug Volcano. You sure can do a lot before taking out Kary, huh? lol

And you don't even have to fight any other fiends. Just go only up in the Sea Shrine and you'll avoid the Water Fiend, and exit the Tower before getting to the Wind Fiend (do it after clearing the floor that has the Adamant in it).