Reshiram learns Fusion Flare and Zekrom learns Fusion Bolt. If you made one of them forget those moves, go to Mistralton City and go to the house next to the Pokemon Center and one of them will make your Pokemon learn that move in exchange of a Heart Scale.
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get the TM and teach it to it
Milotic, Empoleon or Vaporeon.
Inside Drayden's house, Iris will teach you Draco Meteor. In white, Drayden will teach you because you will be fighting iris in the gym.
Teach it to a Pokemon, enter the Pokemon screen, chose the Pokemon that knows fly and select where you want to fly to.
Teach one of your pokemon 'flash' using a TM/HM