You need at least the first badge to use flash.
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you need all badges but only surf is needed. However,for the cave near the end you need strength
First you need the first badge then you need the HM Flash which is given by the oak aide on route 2 lastly go inside rock tunnel and choose the Pokemon that has flash then choose flash the cave will be lighted up more!
my but
You need all eight badges and you have to have beaten the Elite Four in order to capture Rayquaza.
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