You will find Garchompite in the tower near the end of the victory road at night. You can find this tower behind a wall that needs to be destroyed by rock smash. Here's a picture I got from the Internet.
As you can see, the Garchompite is right next to the tower.
Hope I helped and you can look
Here for the source.
Edit : I actually got a better picture. And
Here for another source.