After you battle Hugh in Undella Town, you can continue on to Route 13. Go north through the route gate building to reach the route.
Explore the Route
You might want to bring a pokémon that knows Cut, a pokémon that knows Strength, and a pokémon that knows Surf. There are extra items available on this route if you use those abilities.
Near the entrance of Route 13, Black Belt Wesley is walking around. He has a level 41 Gurdurr.
Up the stairs and to the left you will find Battle Girl Alize. She has a level 41 Scrafty.
Now go north and west. To the south, just before the stairs, there is a tree that you can cut down with Cut. Past that tree, you can find a Max Repel.
Go down the stairs to the left of that small tree, then go south and up the stairs. Use your Dowsing Machine to find a Zinc. Then go north through the grass and use a pokémon that knows Strength to push the boulder up and into the hole. Get the Red Shard to the north.
Go south and go down the stairs, then go slightly north and then east along the narrow beach. Ace Trainer Miguel is walking back and forth in the water. He has a level 41 Delcatty and a level 41 Eelektrik.
Turn on your Dowsing Machine and continue going east to find a hidden Heart Scale. Then get the Ultra Ball that you can see to the right of the big rock nearby.
Keep going east to find Fisherman Leroy, who has a level 40 Vaporeon. There's nothing else over here, so go back to the west along the beach, then north. Going north along the beach you will find Parasol Lady Laura who has a level 40 Castform. Use your Dowsing Machine north of the Parasol Lady to find a Big Pearl.
If you use Surf in the water east of the Parasol Lady, you can reach a grassy island that has dark grass on it and a Luxury Ball in the middle.
North of this small island, you'll find TM29 Psychic in the shallow water. East of that, you find Fisherman Vince, who has a level 40 Golduck. There is nothing else in this area, so Surf to the west until you are on the beach near the Parasol Lady again.
Go west from the Parasol Lady now. There is an Ace Trainer in the water, but he won't battle you. Instead, he'll give you an item. He gives away a random Flute each day. There are no items or anything in the house nearby, but you can talk to the people in there if you want to.
Now go back the way you came until you're near the Parasol Lady once more. Go north from her. The first fisherman is Fisherman Damon who has a level 39 Basculin and a level 39 Staryu. North from him is Fisherman Jones, who has a level 40 Alomomola.
Turn on your Dowsing Machine and go north from Fisherman Jones to find a hidden Pearl in the sand near the stairs.
Now go up the stairs. Go south across the bridge to fight Ace Trainer Angi who has a level 41 Lopunny and a level 41 Lampent. Go south from her to find a Star Piece.
Go north across the bridges. Go west into the path in the trees. There is a hidden grotto in the upper trees at the beginning of this path. This grotto may contain an Eevee, Foongus, Skorupi, or Tangela.
Continue west and go down some stairs. To the south, there is PKMN Ranger Daryl, who challenges you to a rotation battle with his level 39 Jolteon, level 39 Nosepass, and level 39 Galvantula. When you defeat him, he gives you a Sitrus Berry.
Go west from the PKMN Ranger and go into the house there. Talk to the lady with the Wingull to get a Power Lens (Black 2) or Power Band (White 2).
Go back the way you came, up the stairs and through the narrow path in the trees. Go north and you will see some trees that you can get rid of with Cut. Just north of those trees is none other than Cobalion!
If you go east from the trees that you can cut, there is a grassy area where PKMN Ranger Dianne will challenge you to a triple battle with her level 39 Vibrava, level 39 Larvesta, and level 39 Flareon. When you defeat her, she gives you a Sitrus Berry. There is a Blue Shard in the southwest of this area, and a Max Ether up above. If your Dowsing Machine is on, it will point to something to the east, but you can't reach it right now, so just go north and west out of the grassy area.
Now the only thing left to do here in Route 13 is battle Cobalion, who is currently at level 45. Feel free to go northwest into Lacunosa Town to prepare first, although you'll have to talk to Professor Juniper and Bianca when you first go there.
If you knock Cobalion out, you will still get a chance to catch it, but you have to wait until after you have beaten the game.
I don't recommend using your Master Ball on Cobalion.
Cobalion has a very low catch rate, which means that when you throw pokéballs at it, they have a low chance of succeeding, so you will most likely have to throw a lot of pokéballs at it until you catch it.
You can increase your chance of catching Cobalion by lowering its HP as much as possible. If you have a pokémon with False Swipe, you can use that to lower Cobalion's HP to 1.
Also, Litwick/Lampent/Chandelure have very good resistances to Cobalion's attacks, so you might want to use one to lower Cobalion's HP.
You can also inflict Poison, Burn, or Paralysis on Cobalion to improve the chance of pokéball success. Better yet, inflict Sleep or Freeze, because these two status ailments improve the chance of success even more than Poison, Burn, or Paralysis.
The type of pokéball most likely to succeed is a Dusk Ball, but only if the battle began between 8PM and 3:59AM. Otherwise, use Ultra Balls.
The maximum chance of pokéball success on Cobalion (that is, if you inflict Sleep or Freeze, lower Cobalion's HP to 1, start the battle between 8PM and 3:59AM and use Dusk Balls) is only about 8%.
Continue to Lacunosa
Now that you have encountered Cobalion, you can head northwest from it to reach Lacunosa Town.