Mystical Shell is one of the rarest ingredients that you can get in Pokémon Quest. It is impossible to get it on the initial levels. For the first time you can get Mystical Shell on level 12. This raw material may fall out from the boss. If it does not fall out for the first time, you must repeat the level until it occurs. This is the only way to get the first Mystical Shell.
Mystical Shell is needed to create the Ambrosia of Legends a la Cube. It is a dish that gives an opportunity to rise a legendary Pokémon. You can create it with five Mystical Shells, 3 Mystical Shells and Rainbow Matters, or 1 Mystical Shells and 4 random raw materials. Mew, Mewtwo, Moltres, Articuno and Zapados can be cooked from this recipe.