Instructions
Confront every Pokemon trainer you come across to see the Pokemon they keep in their party. Adding Pokemon to the Pokedex this way can save you a lot of leg work. For example, if you battle Swimmer Stan on Route 125 and defeat his Horsea, you can avoid surfing and fishing.
Battle all eight Gym Leaders and the Elite Four to add their rare Pokemon to the Hoenn Pokedex. One instance is Mossdeep City's Gym Leaders Liza and Tate. They use four uncommon Psychic-type Pokemon that may take a player a long time to find otherwise. The Elite Four bring along plenty of hard-to-find creatures as well, including Cradily, Aggron and Sharpedo.
Hunt down legendary Pokemon to add them to the Pokedex. Common Pokemon trainers will not have access to the legendary Pokemon, and it will be up to you to explore their lairs and face them yourself. Groudon, Kyogre, Latias, Latios, Rayquaza, Regice, Regirock and Registeel must be located in the wild to become a part of your Hoenn Pokedex. Kyogre and Latias are the only legendary Pokemon that must be traded to "Pokemon Ruby" to be added to the Pokedex.
Trade Pokemon from "Pokemon Sapphire" if you are having difficulty finding a version-exclusive Pokemon in "Pokemon Ruby." Although many of these monsters are still found by battling enemy trainers, a select batch of Pokemon are not found in the wild anywhere in "Pokemon Ruby." Lotad, Lombre, Ludicolo, Sableye, Seviper, Lunatone, Latias and Kyogre cannot be captured by playing "Pokemon Ruby." Keep an eye out for Pokemon trainers using those monsters in battle.