Fortunately Millennium Labyrinth isn't found in any sort of 'Dungeonception' but via talking to one of the many random hint bearers aka chirpers that you find on the world map. In this case the plan can be obtained from Spelunker at the start of Chapter 5.Upon obtaining the plan for Millennium Labyrinth, you will need to get one item called Magical Reactor 3, which can be found in Naasne Volcano.
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These are all the gameplay videos during my grinding session in Millennium Labyrinth. Be sure to subscribe to my Youtube channel and save my playlist for more Hyperdimension Neptunia Re;Birth1 videos!
Millennium Labyrinth GRINDFEST Part 1
Defeating Tank Type 99 (Tough Enemy)
Millennium Labyrinth GRINDFEST Part 2