After you capture Xerneas (X) or Yveltal (Y) and defeat Lysandre, Shauna says you should go back to Anistar City to resume your journey. After you get there, you should go east from the Pokémon Center. You'll get a Holo Clip from Professor Sycamore asking you to meet him in Couriway Town. Keep going east, then south, to reach Route 18.
To access everything on Route 18, bring a pokémon that knows Cut and Rock Smash.
After passing through Route 18 and its entrances to Terminus Cave, you go south to Couriway Town.
Once your in Couriway Town go west from the wooden platform, then go south toward the bridge. Professor Sycamore appears. He challenges you to a battle. He has a level 50 Venusaur, a level 50 Blastoise, and a level 50 Charizard.
Hope I helped :)