Instructions
Open your PokeGear to view the map. It will display the whereabouts of roaming Pokemon, including Raikou.
Go to Raikou's position on the map. Once you are close to its location, head toward the tall grass and walk around. It will take some time and luck to encounter Raikou.
Attack Raikou with Sleep, Mean Look or Shadow Tag. These moves keep a Pokemon from running away and are usually learned by Psychic- or Dark-type Pokemon. Raikou will attempt to run away when encountered.
Weaken Raikou with a Ground-type Pokemon. Avoid attacking it with Flying-, Steel- or Electric-type Pokemon as Raikou is resistant to these types. Weaken Raikou as much as possible without knocking it out.
Throw a Quick Ball or an Ultra Ball to catch the weakened Raikou.