This achievement is unlocked by finishing each of the 3 secret dungeons through the 2 generations. During Vacation days, there will be opportunities for your male party members to uncover a hidden dungeon for you on the world map. There is 1 dungeon in the first generation and 2 on the second.
First Dungeon:
Cape PromontorioDuring vacation in Alegria Coastline you have to visit the Entrance at night and talk to Tetora.
https://drive.google.com/file/d/1WkzRtNfqZPL2R7vs0Belkp7s3xb...Second Dungeon:
Ohtor PlateauDuring vacation in Mowales Village you have to visit the Adventurer Guild at night.
https://drive.google.com/file/d/10RcdWf_XdVTvEB__RMQRsCCWtrn...Third Dungeon:
Fenstermacher PlainDuring vacation in Will C'est-fini you have to visit the Bar at night.
https://drive.google.com/file/d/1wD5IVJ-A5dbq_Lg2A96sPSyELaz...