Hints and Tips for Super Mario RPG: Legend of the Seven Stars
(SNES)

Fastest way I've found to level up would be inside the Sunken Ship. Continue through the Sunken Ship until you find the first 3 of 6 doors leading to hints on the password. Pass through the next door, and you'll see a person walking around. Its a shop, this is how you will refill your FP. There is also a Dry Bones in there, as you might have found out, Dry Bones never actually dies. So you can fight it over and over again. All you have to do is Use Mallow and Mallow's Thunderbolt. This specific Dry Bones has 4 extra people with it. So after every battle you get 27 xp and around 18 coins. Just equip Mallow with the best armor, weapon, and the Zoom Shoes. This will give him a boost to go before anyone else. If someone is faster than Mallow, take some armor off. If Mallow goes first, use Thunderbolt. Thunderbolt should 1-hit-KO everyone. So in about 10 seconds you can gain 27 xp. Thats over 100xp an minute (around 150 with delay). Since you only need 9999xp for lvl 30. I was got my characters to level 30 in a matter of minutes. It's extremely fast, and very easy to do. I started when my characters were a mere lvl 12.