Successfully complete the indicated task to unlock the corresponding Elemental Sprite:

40 Below: Defeat the Ice Cave Shadow Troll
Ash: Rescue Halis
Chilla: Rescue Lenya
Deep Blue: Defeat the Water Cave Shadow Troll
Flex: Defeat the Sulfur Cave Shadow Troll
Major Rush: Defeat Lobster Hollow Shadow Troll
Pummel Weed: Defeat Enchanted Kingdom Shadow Troll
Rubble: Defeat the Forgotten Forest Shadow Troll
Thermite: Defeat the Ogre Swamp Shadow Troll