Currently, Marshadow is unobtainable in any main series Pokemon game.
Last month, an event in Europe occured where people could get one of 3 Mythical Pokemon, (Genesect, Marshadow, or Volcanion.) This event was exclusive to Europe and ended September 30.
Now, the only way to obtain a Marshadow is through trading or transferring with another player.
Hope I could help!