Successfully complete the game and continue from your cleared saved game file, then explore the surface of the moon until you see a face-shaped rock. Enter it to access a fifty level dungeon. The "Music Player" option will also be unlocked in the "Extra" menu.
When you get to the Underworld for the first time, defeat Golbez in the crystal room. Get access to the armory, then go down to the weapon/armor room. Go behind the pillar that is between the two shop desks. Move up from there to enter the hidden Lali Ho Pub. Then, move to the right to the dark spot on the floor. You can move through the wall from here. Climb down the stairs to reach the hidden 1991 Developer Team Office entrance. Enter the hidden area to talk to the game developers.
When you are in the Developer Team Office, you can find a hidden magazine on the shelf in the first room going down the hallway to the right. If you use it, the screen will turn purple with strange music playing. Then Cecil will say "Now that is some good literature!"
When you are at the Tower Of Zot just before you battle the Magus Sisters, near the save location is a battle that consists of the Puppeteer and the Puppet. If you just kill the Puppets and not the Puppeteer, he will keep summoning back the Puppets. Kill the Puppets but not the Puppeteer to gain lots of experience points. An easier way to level up even more is to kill Cid and Tellah and have Yang and Cecil alive (or any other two characters). You can gain up to 30,000 to 35,000 experience points in five or six minutes.
When you lose everyone after the incident with the sea creature, you will wake up on a coast. Go east and you will enter a town. Once you get to the palace you will get help from two Mages, a black and a white one. After you get them, do not go directly to the place to become a Paladin; train for awhile. This is useful because you will get a lot of Gil and level up quickly.
Note: This trick requires that you be at least over level 25 with all your fighters and be at the Tower of Bable. When in there, there is a team of attackers that consists of a Puppeteer and a Puppet. Kill the Puppets first. The Puppeteer will summon another one. If you are able to kill it in one hit with Cecil, Yang, or Kain, then use only them. Have the others defend. Repeat this as many times as desired, then kill the Puppeteer. You should have obtained a large amount of Gil and experience points. If you fight them, save then leave and look at the Bestiary. You can then see how many are needed to fight to get one of your characters to level up.
To get the best armor in the game, you must get the Pink Tail from the Princess Flan things in the final dungeon. They are encountered rarely, and the tail is an even rarer drop. The easiest way to do this is to first get one Siren from a chest or get one from of an Undergrounder, Arachne, or a Yellow Dragon. Save the game at the beginning of the ruins and find the room with the Flans. Use the Siren and repeat as necessary.
To get through the waterway that requires the Baron Key, do not fight. Just keep running from the enemies to conserve your HP. Make sure you use Dash to get through easily. Note: There will be a Boss after you get through the waterway. When you get in the castle, you must go left or you will reach a dead end. Go all the way to the left and down there will be an opening in the wall. Go through it. If you are hurt or are missing some MP, use an Ether or a Potion. Cecil will be a Paladin by now, but also make sure he is at least level 12. If he is weaker, leave Castle Baron and train Cecil up to level 12. If you decide to fight anyway and are weaker, you should have plenty of Phoenix Downs and Potions. Tellah is very useful by now. He should know every spell in its final form and lower. When you find Baigan, he will join the party but will die later because you have to kill him. Keep casting Firaga on his body. Have Cecil also attack the body. After the body has been destroyed, kill the arms with Fira. Although they are separated, they will still fight. The right arm has a move called Vampire. It will drain lots of HP from you and give it to the other arm or itself. Once you kill one of the leftover arms, it will be easy to win. Note: Make sure Cecil has Paladin Armor or he will get badly hurt.
Cecil and Kain are "brothers". They were raised together and had no family. The king took them under his wing. Kain was not aware, but he always envied Cecil for being with Rosa, whom Kain loved. Kain's trial in the Lunar Ruins is about how Kain's evil spirit is committing murder, though he is not conscious about it. At the end of the trial, Kain has a chance to kill Cecil once and for all, but decides not to and you fight Lunar Bahumat. Once you win, you get a weapon, Kain's strongest one, called the Abel's lance. This is a reference to Cain killing Abel in the bible.