To continue to story and fight Empress, simply enter and leave the hotel in Varanasi 4 times, or use the door inside the hotel to take a rest - the party member you're with will gain 1FP and the event will begin automatically. After a cutscene, you'll take control of Joseph, who is being pursued by police - if you defeat any of the police officers, including the squad car, you'll get an instant Game Over. You can find Jotaro and Kakyoin outside, but when you speak to them, it'll turn out to be someone else entirely and they'll call more police. If you run around the map long enough, an event with Empress will automatically trigger - you can win this battle, but if 10 turns pass or you run away, you'll be kicked back to the map. Go to the alleyway near the Ganges and approach the oil barrels from the right side to defeat Empress and end the event.
Range: S. Must be defeated within 10 turns or the battle will end as if you escaped. Running out of HP WILL result in a Game Over, so it's best not to attempt to fight if Joseph's level is too low - at the very least, he should have Hermit Ripple.