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

How to obtain secret job:-  Onion Swordsman  -  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:

Register someone elses's (or multiple) communication ID under Mog Net.
Send message to another PC 7 times
Send and receive 4 messages from Topapa, on the fourth one you will receive a message to do a mini-quest.
Go to Uoul (First town in the game)
Talk to Topapa
Go to the cave up north of the cave

At the drop hole, you will have an event and then will be proceeded to fight 3 bombs
After the battle, you will obtain a piece of crystal, which will unlock Onion Swordsman

Duplicate Magic  -  *WARNING: This item duplication will ruin the game play, please note to do this at your own risk. If may also damage your save file so please duplicate your save before you attempt this.

In Final Fantasy III, all useable items (not weapons/armors/magic) can be duped using this method, as mentioned, please use at your own risk.

Procedures:

Open menu screen via non-battle.
Select "Items"
Put the item you want to duplicate in the very top left corner in your item list
Enter a battle randomly
When in battle, go select "Item" on any characters.

Your cursor should be at the very top left of the item box, the item you have placed there. Once you're here, hit UP and A button at the same time

This will briefly take you to Weapon/Shield list (about 1/2 second). If you see this, the code has worked after you hit "cancel' or B button. Go to your inventory and you will notice that the item quantity has been added by one.

In Final Fantasy III, all magics can be duped using this method, as mentioned, please use at your own risk.

Procedures:

Open menu screen via non-battle.
Select "Magic"
Go to any character you desire and select by A Button
Once at the magic menu, go select Trade (koukan)
Highlight a character that has magic you want to duplicate, but do not push the accept button (A button). Simply highlight the character (Hover the cursor on the character)

After highlighting the character you wish to duplicate magic off, hit the cancel button once (B button)
You should be back to the first character you chose, now go select Remove (Hazusu)

Now, move your cursor to the level and space of magic you wish to duplicate from the character you highlighted. You will get and hear the error message, but in reality, you are removing the magic that you don't even have (duplicated magic).

*You will know that the glitched worked on begging of step8 if the description of the magic you want to duplicate is describe in the text box. If you are using the character that has magic already (the one you chose on step 3) in the level you want to duplicate magics off, make sure you remove all magic he has equipped on before Step 1, other wise you will rewrite the loose the magic with the one you are trying to duplicate.

Item Duplication(Japanese FFIIIDS)  -  *WARNING: This item duplication will ruin the game play, please note to do this at your own risk. If may also damage your save file so please duplicate your save before you attempt this.

With this method you are able to duplicate any demishable items, even items you can't by such as Chocobo's Rage, Elixir, Phoenix Down.