When you pass the woods and get to Pewter City, You will have to battle Brock the Gym Leader. He uses rock type so Pikachu will be usuless against him as electricity dosn't effect rock. In some of the other cheat codes it says to use water or grass type but you cannot find water or grass types that early in the game. So I caught a Nidoran (girl or boy, it dosn't matter) and trained it untill it learned double kick. Double kick is a fighting attack which is super effective against rock.