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.)
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.
You can't catch Rotom in heartgold.
you cant catch him in heartgold you can only get him in soulsilver
In HeartGold you can catch Kyogre.
You can only catch Vulpix in SoulSilver, not in HeartGold. You can catch it on route 36 and route 37.
In pokemon Heartgold
No
you cant catch it it is rare
You can catch Lombre in Pokemon HeartGold in the Safari Zone at a waterside area once you receive the National Pokedex.
yes you can catch every Pokemon ever made
You can catch Ekans in Pokémon HeartGold in the Safari Zone in the Marshlands area.
there is no Pokemon called croagoak
yes