How do you complete the great maze in ssbb?

To beat the Great maze and unlock the final battle with Tabuu, the ruler of Subspace, you but do both:

Beat the possessed characters

Beat every boss

to find out how many people you need, after beating Dark DK, a bunch of undefeated people with appear. when you defeat them all, a gateway showing Tabuu's wings appear. go to the middle of the room to face the mighty Tabuu