Obtained: After you defeat the boss in the middle of the labyrinth, you will arrive to Block 3, 2F where you will meet Mirage. Talk to her twice.
Much like last time, just work your way through the Idolasphere defeating enemies as you go. When you collect a Shaman Force, Skellington Force and Archer Force, return to Mirage to hand them over and receive a Treasure Key.
Reward: Treasure Key