CM is only available in Black 2 as postgame unless you have another copy of Black 2 as you lose the key when you start a new game. You could also get a copy of White 2 and play CM on it by sending over the key, but for it to be on Black 2 from the start, you will need another copy.
Originally Posted by Moment23
RNG Practice to get perfect shiny legends from BW2
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Pokemon: Pidgey, Woobat, Tropius