Final Fantasy III

Go to the Town Of The Ancients and talk to the child two stories up the grass hills. He is back behind the stairs. He will tell you that he will give you a prize if you ride around the continent. Leave town and go east a bit. Get a Chocobo and ride around the outside of the continent. Go back and talk to the child for your rewards. Sell them, then repeat.

If you want to be able to level up a character to job level 99 in any given job in only a few hours, try the following. Enter a random battle with as weak of an enemy as you can find (since it's arbitrary how strong the creature is). Select the "Guard" command for your characters. Do this for four rounds. On the fifth round, kill the enemy party. Each character will receive a job level. Repeat as needed.

At first I had trouble with the boss but here's an easy way.

Go through the dugeon a couple times to get enough exp to make the battle even easier. But a city to the south called "The Village of the Ancients" contains all three level black magics as well as the white magic teleport.

I used three mages : 2 red (Luneth and Ingus) and a black (Arc). I made Refia a Monk. Have your red mages cast healing and ice spells. A monk can retaliate easily with the new claws found in the dungeon. If you keep the black mage casting ice spells as well you'll definetly win in a few rounds.

Another thing you should probably do is move them to the back, especially the black mage and the red mages too, if they have enough high level magic MP.

To get the Shuriken (available only to the Ninja class for them to throw), head to Gysahl Town. Enter the ship where you can purchase Magic Keys. Go to the right of the counter in the store and you will walk through a secret passage that will allow you to go behind the desk. There you will find the treasure chest which contains the powerful Shuriken.

In the Nintendo DS version of FFIII, there is a secret dungeon that contains a secret boss harder than the last boss, in order to obtain access:

1)Send a total 7 mails to another PC (one or multiple) using Mog Net.
2)Send 4 mails using Mog Net to the NPC: The 4 Elderly
3)Send 4 mails to King Arusu and after reading the fourth one, you'll unlock a quest
4)Go ride on the airship that can go underwater
5)Head for the floating island and once finding it, go exactly south.
6)You will eventually see a shadow in the waters, once you do go underwater to enter the cave
7)Soon after entering the dungeon, you will meet the secret boss

*This boss is notorious and mean. It does a total of 4 attacks per sound and also uses Meteo. Please be at a very high level before attempting this fight.

Back in the original Famicom FFIII, Onion Swordsman was the first job you start as (unlike Freelance from the DS version), but you can play this job in the Nintendo DS version as well by unlocking it. Onion Swordsman is one of the most powerful jobs with the Onion Equipment + past level 90's.

How to obtain:
1)Register someone elses's (or multiple) communication ID under Mog Net.
2)Send message to another PC 7 times
3)Send and receive 4 messages from Topapa, on the fourth one you will receive a message to do a mini-quest.
4)Go to Uoul (First town in the game)
5)Talk to Topapa
6)Go to the cave up north of the cave
7)At the drop hole, you will have an event and then will be proceeded to fight 3 bombs
8)After the battle, you will obtain a piece of crystal, which will unlock Onion Swordsman

There are six level eight spells (Flare, Death, Meteor, Tornado, Arise, Holy), but they can only be purchased for use at one shop. The shop is located in the Forbidden Land of Eureka, and each spell can be purchased as many times as you'd like for 60,000 Gil per spell.

In the earlier parts of the game when you are alone and party-less, excessive levelling is much recommended since the level at which your comrades will be introduced to you will highly depend on the level you are in when you meet them.

For example, Fran and Balthier will always be 2 to 4 levels above yours when you get to use them. If you're Lv10 then they can be anywhere from Lv12 to Lv14. This may seem unimportant but it'll soon become evident in the higher levels when each level requires 30K exp and ontop of that experience points from kills are split to each of your party member. These small early foundations can become a godsent farther in the game.

For the Onion Knight, this weapon can be found in the secret dungeon by slaying Red Dragon.

If you get the Onion Knight, there are special weapons and armor that only he can equip. They are called the Onion Sword, Shield, Gauntlets, Helm, etc. You can get them by fighting red and green dragons. These can be found towards the top of the Crystal Tower and in the "secret dungeon".