In my opinion, it would probably have to be Mewtwo. I mean there might be harder things out there, but Mewtwo was the biggest challenge I had ever faced in Soul Silver. (NOTE: HEART GOLD AND SOUL SILVER IS BASICALLY THE SAME THINGS. THE ONLY DIFFERENCES ARE NAMES OF THE GAMES AND VERSION EXCLUSIVE POKEMON.)
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You can catch Lombre in Pokemon HeartGold in the Safari Zone at a waterside area once you receive the National Pokedex.
Find and then catch
You can't catch it. It's a starter pokemon.
you do not catch it . you can bulbasaur when you get it from proff oak
you can catch a growlithe at route 37.