Use the UMN again, and select the second option which is MOMO's Encephalon (Summer). 
Head to the right for the next area. Continue to the right to the next area. Now in 
this area, go right a bit, then NW for a little house. Destroy it and reveal yet 
another Forbidden Device. With the Sequencer C, dive into the Encephalon for the 
second secret dungeon. Defeat the boss in this area to activate an elevator that 
leads you to the end area of the secret dungeon. Within this area, you will need 
to activate G2 Campaign No.21 to do the puzzle here. Clear the puzzle and get the 
Mufufu Book along with Sequencer A.