You need to use the Itemfinder in every walkable/surfable spot inside without getting encounters (you cannot save and reset and you cannot use repels). When you use it on the last spot you'll get the Mist Stone, which you can use to evolve Pikachu into Pikablu. After defeating the Pokemon League 365 times with a party consisting on the entire Caterpie and Weedle lines, you'll need to do it once again a 366th time but with only Pikablu in your party. This will allow you to enter the Pokegod City. Source: My uncle works at Nintendo.