You may have to train your Pokemon to at least level 20 or use a electric type like pikachu you can catch it in viridian city but its rare to find.
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With Pokemon ---- No seriously though , just get a bellsprout and train it a bit..( lvl 18)
Once you fixed the Power Plant, go to the end of Route 25. Talk to Misty and she'll return to the gym. Then you can beat her. (She uses water Pokemon so I suggest you use grass or electric Pokemon.)
Beat misty then go to cerulean cape
You will only need to beat it if you want to transfer pokemon from the games Ruby, Sapphire, Emerald, Pokemon Colosseum and Pokemon XD Gale of Darkness. You can transfer pokemon immediately to another leafgreen or a firered version using either the GBA link cable or the Wireless Adapters.
He will appear after you beat the 7th gym leader.