Normally during the game, you cannot progress into B26F to B29F of the Labyrinth in Story Mode. Once you beat the game on B30F of the Labyrinth and view the credits, do not opt to start a new game at the title screen. Instead, opt to Continue your game: after leaving the inn, you will be notified that these four extra floors are now open to you. In addition, you will have Kupala back in the guild house and the Yggdrasil Core will not be weakened on B30F when you fight it again by any of the means used the first time. You can access the staircase to B26F on B25F: there, examine the Geomagnetic Pole. Once the Pole disappears, stand where it was, face south, and press A to find the hidden stairs.