i really hope you need to know this to complete your dex, otherwise, just train, EV yields are completely worth it. also, if you're training i would recommend trading your Pokemon over to a different game, that way they get boosted exp.
if you want to easy way out though, rare candy. you won't get EVs so hopefully strong Pokemon don't matter to you.
get a rare candy.
give it to a Pokemon to hold, use the cloning glitch, if you have emerald. it's all over the internet, very easy to find. there you go. lots of rare candy.
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the easiest way to level up your Pokemon to put it in daycare Pokemon will level up soon.Trade the Pokemon to friend and have it hold a lucky egg it will level up quickly.
the easiest way for me is to give it exp.share and face the elite 4
The easiest way to level up is to feed Pokemon Rare candies.
Either cheat or "chain" 40 pokemon.
The same way you would catch a normal Pokemon. The only difference is is that the Pokemon is shiny.