After you finish the 2nd Pokemon Gym in Eterna, use cut on the trees around the fence near Eterna Forest exit. You will find the Old Chatue and all there is in there is Ghastlys and maybe Haunters. Go to the room with the Tv, and make sure it is Night Time (20:00-4:00), then go ahead and "thump" the Tv and Rotom will come out so you can catch it.