You'll have to go quite a long way back; all the way to Gen III.
In FireRed and LeafGreen, you have to take a Salamence to a Move Tutor and learn it Body Slam there. Then, you need to transfer it all the way to, I'm supposing X and Y. You will need a Gen III cartridge, a Gen IV cartridge, a Gen V cartridge and a Gen VI cartridge... good luck.
Hope I helped. :)