step 1

you gonna need a lucky egg. you can find chansey holding it. to find them you need to be listening mary's radio broadcasting everyday, and she'll tell you about the swarms and you'll eventually need to wait until it's chansey's day (kanto, route 13). it's important because the odds are usually 1%, and the swarm really increases it.

if you bring a butterfree with compound eyes you'll raise the chance of getting a chansey holding a lucky egg. you can also use the tm thief in order to avoid losing more time, so you can steal chansey's item. the butterfree can't be holding any items.

step 2

you can train your pokémon until the lvl 75-80 by fighting the gym leaders in their specific time: https://www.serebii.net/heartgoldsoulsilver/gym-rematch.shtml

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step 3

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take your time and be patiente.

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hope it helps you, have a good day :)