You'll need to go into Mistralton Cave and somewhere in the far back. (Requires going upstairs, and plus Surf, and Flash.) I can't remember the direction, but I hope someone else does to help you out.
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You can catch the main white legendary, Kyrugem, Virizion, Cobalion, Terrakion, Thunderous, Victini, Zorua, Zoroark, Volcarona, and Landorus.
you catch cobalion
Mistralton cave on route 6.
Leave and return to the cave and Cobalion will have returned. So then you may capture it. I suggest Timer Balls.
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