Speak to Yang's wife as soon as you get out of the Underground and she will give you a Frying pan. Use the frying pan on Yang in the cave with the Sylphs. He will get knocked conscious, but he will not join you. Then, give the frying pan back to the Wife and she will give you a Spoon. The Spoon is the most powerful throwing weapon for the Ninja.
Unlock the following chapters by performing the corresponding tasks:
CHAPTER - HOW TO UNLOCK Kain's Tale "Return of the Dragoon" - Complete Ceodore's Tale and any other character's tale. The Lunarians' Tale "The Blue Planet That Was" - Complete every tale up to and including Kain's. The Crystals "Planet Eater" - Complete every tale up to and including the Lunarians'.
Equip Cecil with The Crystal Sword, Helmet, and shield, then equip him with the Adamant Armor and the Zeus Gauntlet (glove). Make Cecil use the CRYSTAL to transform Zemus to Zeromous. Use Edge's Spoon dart and regular attacks, then use Kain's jump attack or White spear. Make Rydia cast METEO, NUKE, and BAHAMUT. Make Rosa cure the party and use the WHITE spell to hurt Zeromous bad. Repeat this about 4 times and Zeromous will die.
After defeating Calbrena and Golbez in the Dwarven Castle, you will get Rydia back. When King Giott finishes speaking to you, cast the Warp spell. You will appear in the Crystal Room behind the throne and the Crystal is strangely still there! Take it and this will save you the trip to the Sealed Cave. You will still have to take Luca's Key and enter the exit the Cave, but then Kain will take the Crystal as if you defeated the Evil Wall for it.
To kill undead enemies (ie. spirits, skeletons, etc.), use cure on them. Fire works just as well, but if your character can't use magic, equip him with a holy weapon (i.e. Light Sword, Crystal Sword, etc.)
Equip a Cursed Ring along with a piece of armor that protects against a particular element (i.e., Diamond Armor protects against Thunder/Lightning), and you will absorb the damage anytime that element is used against you. This is especially useful in the endgame, when you get armor such as the dragon and crystal armor, which defend against all elements. Be careful though. It greatly reduces your physical defense.
In the Cave of Summons, there’s a monster within called a Summoner that summons certain monsters. Let the creature summon its monsters and kill only the monsters, but not the Summoner itself. Do this long enough to earn crazy amounts of experience by killing hundreds of summoned creatures before killing the Summoner itself.
In the underground, go to the land of monsters and get the rat's tail in a chest near the entrance. Then, in the overworld, use the HoverCraft to get to the silver mine close to Silvara. Give the guy the rat's tail and he will give you Adamant. Return to the underground and find the weapon master in the lower-right hand corner. Speak to the guy on the second floor and he'll transform your old Legend Sword into the super-powerful Excalbur. NOTE: Only Cecil can use the Excalbur.
Equip Cecil with arrows in his good hand and a bow in his bad, then get in a fight. Go to items, select the arrows he has, and put them in an empty space. Without leaving the window, select the Avenger sword and move up and trade it for the bow you had equipped. Leave the item window, and his commands will disappear. Do NOT have anyone else do anything! Once Cecil has taken out all the enemies, go to the EQUIP screen and you should see the Avenger in his bad hand and a number in his good hand. Unequip the Avenger, and equip whatever weapon you want duplicated in his good hand (where the number is). De-equip, then re-equip. Voila! The weapon in the item screen will have a black box and a number beside it. 255 of them!