Go to the first Secret Dungeon - Desert and go all the way back to the room with an item chest and a Forbidden Device. To the north of this room, you can get off your E.S. Continue along the path. With the Sequencer D, you can dive into another secret location, the Space Coliseum. In here, talk with Black Hakase/Professor to trigger an event and eventually you will end up fighting Black Erde Kasier.