Using the Map
The map for the Great Maze is found on your sub-screen by pressing the pause button and going to the FAQs section. The map is comprised of several places you traveled to throughout the game, including Skyworld, the Glacials, the Ruins, the Sea of Clouds, the Swamp and Halberd. When viewing the map, the green dots are doors and the white lines next to them show where the doors lead. If there is a dot without a line, there is a dark character behind that door. The dark characters are actually found in the same areas where you battled their light equivalents earlier, (like Pit in Skyworld, Ice Climbers in the Glacials and Bowser in the Cave). Whenever you reach a dark character's location, you will find a door covered in black smoke; enter it to find the dark character. In the very center of the maze, there is a room with a giant, locked, purple door and a trophy case. Whenever you defeat a dark character, that character's trophy disappears from the case.
Clearing the Way to Tabuu
Travel across the entire Great Maze in a circle, find all the black doors and defeat the characters behind them. The maze itself is straightforward except for one place in the Jungle where you must destroy blocks to reach some of the doors. Also, some areas can't be reached until you beat certain characters; Zelda clears the way to the Sea of Clouds, and beating Galleom gets you to the Ruins. Once you have defeated all of the dark clones of your characters, six more foes will appear on the map; their positions are shown by Bowser heads. These are clones of the six boss characters you defeated earlier in the game. All in all, you must beat 37 opponents in the Great Maze. Once you have beaten all of them, return to the trophy room with the purple door in the middle of the maze. The trophy case has been cleaned out and the door is now open. Enter the door to face the final boss, Tabuu. You get a choice of five characters to team up against him; since he is in the air most of the time, look for characters with air attacks like Peach, Marth and Diddy Kong. He has several different attacks, so stay in the air and hit him there as often as you can.