Keep Kenya the Spearow
When you get the Kenya the Spearow with mail to deliver to the person on Route 31, take the mail, and then attach it to any other Pokemon. When you deliver the mail, the person will say they don't know the Pokemon, but will accept the mail anyhow. Since you are not Kenya's OT, you get 1.5 times EXP, as though you got her in a trade.
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