Thats the midnight scene or does the last scene with Mei come too?
Last scene with Mei is after you do all her missions (destroying, capturing and giving her back robots). To get Midnight last scene you need to do mall resque. Sens ejiro alone. Recomending sending ochaco as scout. If Mei as robot comes to you, select to look for Midnight. After this, sleep and go to bathroom. Midnight Will be on shower. After short dialogue go to your room and again to bathroom. Then go to bedroom with her and ready