There is no such Pokemon as "Shadow" mewtwo. There is a regular mewtwo that you can catch like any other Pokemon. It's located in the Cerulean Caves.
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you catch it in cerulean cave
nothing. the game is over. congratulations. (well, you can go to cianwood city and enter the cave to catch groudon and rayquaza.)
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't catch charizard in Pokemon soul silver. You can get a charmander from professor oak.
sorry you have to trade it to Pokemon soul silver