I've beaten the game, gone to Kiluode and battled my rival, and upgraded my Mega Ring by the Sundial with Prof. Syccamore, I've also gone to the location between 8-9 PM, but it still won't show up. What am I doing wrong?
This video talks about it.
You would have have to beat the Elite Four. You would also need Rock Smash.
I haven't looked through the whole video, since I don't want it to be spoiled, but by the looks of it it looks trustworthy.
Hope I helped. :)
Source: Above Link
If you've gone to it's exact location on Victory Road, outside by an abandoned tower behind a smashable wall, you should see several sparkles on the ground where the garchompite is. It should be right infront of the tower.