Players will have to play the overall game from March 24th to March 29th to get a Shiny Rainy Castform. If you don't have time to play games, then Buy Pokemon is another best option. Fortunately, the spawn rate with this Pokemon will probably be greatly increased. To save on time, tap over a Rainy Castform about the overworld if it's not shiny, hightail it. There is no real-time catching it until you want that additional Stardust. Players also can use an Incense or Lure Module to raise the overall spawn rates of Pokemon.