Adventurer Corey is the second Mini-Boss in Mount Gemma. She has a level 28 miscrit team and they are Tesla fish (Jellyshock Evolution 3), Zephelope (Aria Evolution 3), Gemphibian (Croaky Evolution 3), and Badaboon (Thundercracker Evolution 3). She bars the way to the Earth Elementum (Skyline Peak and Crystal Cliffs).
Arrange your team so they always have elemental advantage. This is a fairly straight forward fight. I used a level 28 team and had little trouble. Your team sould be arranged Earth, Lightning, wind, and earth (or just swap your earth miscrit back in during the fight with Badaboon.