exluding level 70 legendaries, i think its around 55, maybe 56. stark mountain would be your best bet for the highest levelled wild pokemon.
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You can't get it, unless you train a Pokemon to level 100. The highest level wild Pokemon is arceus which is lv80, and you need an event for that, but without and event you can only get an lv47 Dalgia/Palkia for the highest level wild Pokemon.
It is not possible to catch a wild Turtwig on Pokemon Diamond. It is generally impossible to find and catch Starter Pokemon in the wild. This was a mechanic designed to encourage player interaction for trading.
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Use poke'nav and get it in wild
u cant Pokemon bloody diamond isn't real hoped this works :)