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So, it turns out that in Crystal's postgame, you get access to a trainer with a lvl 41 Chansey that you can easily rebattle every 2-3 minutes. Pokemaniac Brent on Route 43 switches up his team for every rematch, going from a Lickitung to a Kangaskhan to a Porygon and ending up with the aforementioned Chansey. You can easily make him call you for a rematch by switching Daylight Saving Time on and off with your mom with him as your only phone contact, and a lvl 41 trainer-owned Chansey is probably the best exp in the game outside of maybe some of the gym leaders. I'm probably not the first to figure this out, but I haven't seen anyone else talk about this, and this is probably the best postgame grinding method out there (but sadly only available, as mentioned, in the postgame, specifically after delivering the Machine Part)
