1. The strategy here is simple: you need to make sure that all rare artifacts are obtained at the bottom of the duel locations. And also buy everything that is in stores (except for seller L’s renewable goods and books for resetting parameters and skills). If at the bottom of the location there are only artifact icons with an infinity symbol, then all rare artifacts have already been obtained.
This particular artifact can be found in the desert.
2. This part of the plot has not yet been fully revealed in the game's plot. But people from the guild usually help with the initial stage.
Listen to the Bartender's rumors.