1. Erase all of the moves except for Leer from your Shining Gyarados.

2. Now trade it to a Blue/Red or Yellow Version.

3. Now teach it Mimic.

4. Make sure you have either Ultra Balls or a master ball.

5. Now find a wild Ditto.

6. Use Mimic and learn Transform.

7. Let it transform into you and then catch it.

8. Now trade it back and you now have a Shining Ditto.