You must have completed Treasure Island, Ahoy! in order for this quest to show up.
Once again we are with Rikku on her hunt for treasures. But this time she’s super irritated due to the lack of actual chests. Watch the scene and get ready for a couple fights.
We fight two different battles here. The first is against a Mimic who has no elemental weaknesses or strengths. However, right after that battle we are thrown into another one. The second fight requires Mirages that use Light if you have them. When I went into it I didn’t have any Mirage’s that came with the Banish (light magic) ability. You should get a Banish Seed throughout your journey so have one of your stacks equipped with it.
Battle vs. Mimic Queen
Hp – 5740
Elemental – +50 Thunder / – 50 Light
Stick to Light attacks and feel free to use whatever stacks you prefer. Stay away from Lightning since it won’t do much damage to the Queen. Champion Medals are good things to use as well.