Instructions
Travel to routes 13, 14 and 15.
Walk in the tall grass until a Chansey is encountered.
Use your Pokemon's moves to lower Chansey's health bar. Do not use Fighting type moves, as Chansey is weak against these and you may defeat it by accident.
Use any status changing moves on Chansey, such as Sing or Stun Spore, to make it easier to catch.
Use an Ultra Ball or another strong pokeball, to capture Chansey when its health bar is red and it has been weakened by a status changing move. If Chansey is not caught the first time, continue to use Pokeballs until it is caught.