After rescuing the legendary pokémon of Kalos and defeating Team Flare once and for all, go southeast from Anistar to Route 18.
Battle and Catch
To access everything in this route, bring a pokémon that knows Cut and Rock Smash.
In the grass and flowers, you might find Sandslash, Graveler, Heatmor, Durant, Torkoal, Gurdurr, Pupitar (Y only) or Lairon (X only). In addition, you might have a horde encounter with Geodude, Heatmor, or Durant. Also, if you use Rock Smash on the rocks in this area, you might encounter Graveler, or in rare cases, Shuckle.
Go south from the gatehouse and turn on your Dowsing Machine to find a hidden Poké Ball to the right of the Route 18 sign.
Go south from there and hop down the ledge to find a Hyper Potion. Then go east and north through the grass.
Nearby, you find Youngster Jayden. He has a level 48 Scolipede and a level 48 Pinsir.
Go east from here, then use Cut on the pricky tree before the bridge. Go north to find a Wacan Berry tree.
There is a skating rail to the right. Use it to find a PP Up. Then go south, over the ledge, and south through the grass.
Go up the stairs. At the top of the stairs, you find Hiker Orestes. He has a level 50 Stunfisk.
The cave entrance to the right of the house is the main entrance to Terminus Cave. This is a big area that you can explore now if you wish, or come back later. It's optional. There are new pokémon in Terminus Cave that you can catch, and there are trainers inside that you can battle. Also, after you beat the game, you can catch a legendary pokémon in Terminus Cave.
To the left of the cave entrance is a house. Go inside and talk to the guy on the left. He is Psychic Inver, and you can have Inverse Battles with him, once per day. In an inverse battle, type effectiveness is reversed. For example, in normal battles, Water is supereffective against Fire. In an inverse battle, water is not very effective against fire.
Use the Dowsing Machine west of the house to find a hidden Ether.
Running back and forth on the bridge is Battle Girl Justine, who has a level 52 Mienshao.
There are some train tracks south of the cave entrance that is to the right of the house. If you walk south along the tracks, you find Sky Trainer Jeremy, who has a level 46 Ninjask, a level 47 Drifloon, and a level 50 Flygon. Remember: only your Flying-type pokémon can participate in a Sky Battle.
If you go slightly north from the Sky Trainer, then east along the tracks, go south to get a Max Ether.
Go into the cave entrance north of where you got that Max Ether, then go east, north, and west to get TM31 Brick Break. This cave doesn't connect to the rest of Terminus Cave.
Now go down the stairs south of the Inverse House. Go southeast and use your Dowsing Machine to find a hidden Super Potion on a rock near the broken tracks.
Go south from here and use Rock Smash on the rock in the middle of the tracks. Then go east and walk under the raised tracks. Go north. There is a rock under the tracks that you can smash with Rock Smash, just under the intersection of the two sections of raised tracks.
Keep going north. Black Belt Yanis is running around. He has a level 49 Sawk and a level 50 Pangoro.
East of the Black Belt, there is an X Defense.
Go into the cave entrance near the Black Belt to find a Dragon Scale. A Seadra holding a Dragon Scale will evolve into Kingdra if traded.
Still in the cave, use your Dowsing Machine to find a hidden Normal Gem in the north end of the cave. If a pokémon holds a Normal Gem and uses a Normal-type move, the move will be 50% stronger, and the Normal Gem will disappear after the move is used.
Go back outside. Go southwest to the area of red flowers. Turn on your Dowsing Machine to find a hidden Honey in the northeast corner of the red flowers.
To the west, you find Lass Sara. She has a level 48 Granbull and a level 48 Dedenne.
Go south from there and down the stairs to enter Couriway Town.