Instructions
Challenge and defeat every Pokemon trainer you find. These rivals use a variety of monsters in battle, and battling the challenger's Pokemon is how you will fill in the majority of the pages of the Pokedex. For example, if you battle Super Nerd Glenn on Route 8 and defeat his two Pokemon, Grimer and Muk, you will not have to search out these slimy creatures in the wild to add them to the Pokedex.
Seek out Pokemon gyms and take on the trainers and gym leader inside. The gym leaders sometimes utilize Pokemon that cannot be found in the wild, such as Celadon City's Erika. Her Victreebel and Vileplume are examples of grass-type Pokemon that are not found by wandering the wilds of Kanto.
Confront the "Elite Four" after beating all eight gym leaders to find a few exotic Pokemon that are difficult to find outside of a Pokemon battle. Examples include Lance's Dragonite, Bruno's Machamp and Agatha's Gengar.
Explore off the beaten path to find legendary Pokemon. A few Pokemon will never be added to the Pokedex by battling trainers, so you will need to search for them instead. Articuno, Zapdos, Moltres and Mewtwo are all legends that need to be encountered in the wild to be added to the Pokedex.