This is one of the Intervention quests you'll have access to through the Girl Who Forgot Her Name in her tea room. To unlock this, you must have Coliseum access as well as have gone through Babil far enough to unlock Tifa as a Champion. The intervention quest you want is "A Duo of Three". You'll go through three rounds of fighting Mirages but they should be of no concern if you've made it far enough to reach Nibelheim with little concern. Watch the story unfold and you should be rewarded with this trophy after all scenes play out.