There's a logical explanation for everything in this game. Well, except for the joke with the fisherman and Morden. When dealing with Mirania, as with any other boss, you need to find the optimal method for defeating her. This will determine the speed of your victory. For example, Violet is a good shield against Lamia's magic. But she needs to keep her body well hidden.
You can usually find out about the characters' locations from the fortune teller, the bartender, or a marker on the world map, depending on the situation.