Monster Maze

Monster Maze unlocks at level 8. You can get Pokemon shards, mega evolution stones and fruits.
Do note that you will only get 5 shards for Mewtwo X and Mewtwo Y. You will only get 10 shards for Groudon, Kyogre and Rayquaza. This is something you should do daily and it's recommended to go for the Mewtwo X or Mewtwo Y dungeon since they are strong free to play Pokemon.