First you have to have a Victini in your party. Make sure that the Victini in your party is either from Liberty Garden or an event one. If it is a Victini obtained by using cheats, it won't work. After you have fulfilled these conditions, put the Victini in front of your party and talk to that lady, she will turn into a Zoroark level 25 or 30 and battle you
Hope I helped?