This won't complete it, but it will help tremendously. Don't skip trainers! Fight every trainer you see. Many of them have Pokemon that no other trainer in the game have. Also, when you come to an area you have never been to before, don't use any Repel. You may run into a Pokemon you haven't seen before.
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First complete the sinnoh pokedex, 210 Pokemon
That pokemon is Azumarill in pokemon platinum in the national pokedex
you need to find 210 Pokemon to complete the pokedex
all of them in the regional pokedex
Pokemon Platinum has two different Pokedexes: the Sinnoh dex and the National Pokedex. In the Sinnoh dex, #152 is Rotom. In the National Pokedex, it's Chikorita.