To fight The Empress, kill the Bonewolf in the Stephanie and Bonewolf fight. After first encountering Stephanie and Bonewolf, you can fight them again at the laptop in the Refuge (starting area) instead of waiting for a random encounter. If unlocked, use the Pride modifier (found randomly in dark grey boxes in shop areas) and avoid getting hit to guarantee an M rank upon victory. Otherwise you can increase rank via "stylish" moves such as aerial kills, dodging, parrying, and dealing damage without getting hit.