Question

How can I get a Salamence with Body Slam?

Answer 1

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. :)
Source

Answer 2

Through the Gen III move tutor.

In Leafgreen. In the Battle Frontier, in the left hut, A move tutor (the old lady) will offer to teach Body Slam for 48 BP.

Hope I helped!