The following is a walkthough of the end of the World of Balance, better known as the collapse of the world (referred to as this later by Celes when talking to Setzer in Kohlingen).This is a guide for after you get to the save point/airship. You can return on your airship but if you'll have to start from the beginning again. For this event, I ultimately chose Locke, Terra and Setzer. Locke is level 51, Terra, 54 and Setzer 46. They all know all magic up to this point. If you want battles to go way easier on you, have the following relics on your characters: Black Belts on all characters and if you were able to get two Genji Gloves, equip them. On the other two characters equip a Ribbon and perhaps Sprint Shoes. You definitely want the best equipment and armor.
Following the save point, hit the switch in the upper left corner and go through the portal that opens. Pass the airship until you can go no further and face Atma. Atma is a boss not to be taken lightly. Make sure you keep your characters healed, especially if one has the Atma Weapon equipped. You can steal a Ribbon from Atma. I put Terra in her Morph form after stealing the Ribbon and finished the battle in a disappointing three minutes. When you're done you'll see a cutscene of Kefka moving the Goddesses and thus throwing everything out f whack, and he also kills Gestahl. Shadow will leave your party but will be replaced by Celes. If she's already in your party you'll be left with only three characters.
Make sure to equip Celes before doing anything else. You can also pick up a couple of items on your way, Don't worry about timing; you have five minutes and when you get to the end you'll have to wait for Shadow until 0:05. If you don't wait for Shadow, you will NEVER see him again. The only enemy you face is Naughty, and the boss Nerapa. Choose to wait for Shadow and he'll be there when you have five seconds.
On that day, the world was changed forever...