Well, you need to be on a New Game Plus, and select the optional boss bonus when starting the new game. On Wednesday, a Free Battle option called 'Original Wife' will appear. In this, Lilith has charmed many male citizens, all equipped with demons. The goal is to kill her without killing any of the citizens. She herself is pretty tough, with an ability that charms all male targets without protection and can even work on females as well. Beating her unlocks her for fusion.