The Black Belt seems weak at first, but when you get him to level 10, watch his attack strength sky rocket! The Black Belt's DAMAGE rating is always equal to double his level, unless he has a weapon equipped. So basically, don't equip any weapons on him. Same goes for his defense. His ABSORB is always equal to his level unless he has armor equipped. So when you get him to level 50, his ABSORB will be 50, which is pretty good considering the highest a Knight can get is 74, so the Black Belt/Master is the best character to have. He requires no armor or weapons.
To find Master Unne, the professor who studies Lefineish, obtain the Slab from Kraken's Castle Underwater. To understand the Lefineish, Master Unne MUST decipher the Slab. Take the Slab to the village that the Vampire destroyed the clinic in. (If you are confused, it's the first town you go to after Nerdrick destroys the rock with TNT. ) He is in the Upper-left corner of town, by the graveyard. Once he deciphers the Slab, head to the Lefineish's Village!
Get the ship and save at the first town (30gp INN). Save and turn the device off. Don't reset or it won't work. Turn it back on and restart your game, then head south and back to you ship. Keep sailing south and you should get attacked by KYSOKUS, the blue pirates. These will always attack you if you follow this process, allowing you to gain 2000gp in about 4-5 fights. You can then buy the good LV 4 magics and good weapons/armor to make leveling up easier before going after the crown.
In the sky castle, there is a long bridge just before fighting Tiamat. On this bridge it is possible to fight an enemy even more powerful than Tiamat herself, called Warmech. Warmech has 1200 HP and can cast some mean spells. If you encounter it, just use Nuke and fight with your Master Knight or Ninja. Heal with the white mage. The warmech is a powerful foe and fighting him is optional. If you can kill him than you truly are a Final Fantasy Master.
For quick experience at the start of the game go to Astos castle. After you get the Mystic key you can go into the treasure room. There are three chests and in front of each one there is always a fight. It is usually either Mummy's or Images. If you have Harm2 and Fire2 you can defeat them without much trouble. Each time you step in front of a chest there is a fight so you can get quick experience. You get roughly 60 ex. for each Image and 75 ex. for each Mummy. They come in packs of 3-6 and give good money too.
While you are in the Sky Castle (Level One), you will find a chest. Inside is the Bane Sword and used as an item, the sword will cast Bane. Use the sword as an item and Tiamat can be defeated in one round.