After reaching Dendemille Town, you find that the eastern road can't be crossed because Mamoswine is at Frost Cavern.
Battle and Catch
Near where you enter Frost Cavern, you find Hiker Ross, who has a level 44 Vibrava.
Go east from the hiker. Just south of the bridge, Sky Trainer Celso is standing on a hill. He has a level 41 Carnivine and a level 44 Swanna.
Go north across the bridge. You find the Mamoswine from Route 17, who seems worried about something in Frost Cavern.
Go east from here and walk to the eastern edge of the cliff. You will find Sky Trainer Era standing across the chasm. She has a level 41 Cryogonal.
Go back to where the Mamoswine is, and continue northwest. You find Artist Salvador, who has a level 44 Smeargle.
Use your Dowsing Machine near the artist to find a hidden X Sp. Def.
Go north from here. There is a Photo Spot where you can get your picture taken in front of the mountain.
Turn on your Dowsing Machine and go north. There is a hidden Escape Rope to the west of the cave entrance, but you have to walk diagonally to get it.
Frost Cavern 1F
In the cave, you might encounter Jynx, Piloswine, Beartic, Bergmite, Haunter, and (rarely) Cryogonal. You might also have horde encounters with Smoochum, Vanillite, or Cubchoo.
Now go into the cave. Use your dowsing machine and go west from the entrance. There is a hidden Ice Heal on the rock.
Go east and you find Ace Trainer Cordelia running around. She has a level 46 Doublade.
To the east of her, there is some ice. Once you start walking on it, you'll slide until you bump into a wall. Slide east, then north.
After going up some stairs, go north to some stairs leading up. This is a dead end, but use your Dowsing Machine to find a hidden Dire Hit on the rock. Go south and down the stairs.
Go east and slide across the ice. If you slide across to the upper part, use your Dowsing Machine in the middle to find a Dusk Ball on the north wall. Then slide east the rest of the way. You can walk behind the Ace Trainer to avoid the battle if you wish. Ace Trainer Neil has a level 42 Raichu, level 42 Golduck, and level 43 Marowak.
Go north from the Ace Trainer and up the stairs.
Frost Cavern 2F
Slide north on the ice. In the space where you end up, use your Dowsing Machine to find a hidden Ice Heal on the rock to the right.
Walk all the way to the right if you aren't there already, then slide south on the ice. From there, slide east, south, west, and north.
You end up on some solid ground. Slide west. If you slide west from the middle of this area, you can avoid a battle with Hiker Delmon if you wish. He has a level 42 Relicanth and a level 42 Rhydon.
Stand just to the east of Hiker Delmon and slide north on the ice to find an Ice Heal. Then slide back south to where the Hiker is.
Now stand to the west of the Hiker and slide south. You should bump into a large rock. Slide west from there. Now you can walk west.
The path splits. If you go north, you'll encounter Black Belt Alonzo, who has a level 43 Scrafty and a level 44 Throh. If you go west instead of north, you run into Brains & Brawn Eoin & Wolf, who challenge you to a double battle. They send out a level 44 Grumpig and a level 46 Hariyama.
South of Black Belt Alonzo, you find Battle Girl Kinsey, who has a level 43 Sawk and a level 44 Mienshao.
There is some water here. If you Surf on it, you might encounter Poliwhirl, Lombre, or Floatzel. If you fish with the Old Rod, you'll find Poliwag. With the Good Rod or Super Rod, you might encounter Poliwhirl and Basculin.
If you use Surf to go west from here, you find a path that leads to some stairs leading down. In the downstairs area, there is an Ice Rock that you can use to evolve Eevee into Glaceon. Just have an Eevee in your party and level it up in this room.
Go southeast of the ice rock to find an Icy Rock. If a pokémon holds it, the duration of the Hail move will be 8 moves instead of the usual 5.
Go back upstairs and Surf east across the water.
To the south, there is more ice to slide down. There is a rock in the middle of the ice. Go to the left of the rock and slide south. Turn on your Dowsing Machine and go south to find a hidden Pearl on the rock.
Now go to the left and slide to the left on the thin patch of ice. From there, slide south, then east and south. Go to the left side of the solid ground that you end up on, then slide north, then west. Slide west again to reach the water. Surf south and you will find a Never-Melt Ice. If a pokémon holds this item, its Ice-type moves will be stronger.
Surf north again, and slide east. Slide east again from the narrow strip of snow to reach a Max Repel.
Slide west, then when you are on the narrow strip of snow, walk to the south end of it and slide east. You should bump into a rock. Slide south from there, then keep going south to reach some stairs going down.
Frost Cavern 1F, part 2
Go south and talk to the Ace Trainer (she doesn't battle you) to heal up your party. (She mentions a bivouac, which is just a temporary camp.)
Go south from her to find a Hyper Potion.
There is a ramp to the east that you can use to quickly go back to the entrance to the cave, but it's one-way, so you would have to take the long way to get back up onto this ledge.
When you go north, you can avoid a battle by staying on the right side of the path. Up north you'll find Hiker Alain, who has a level 40 Graveler, a level 41 Graveler, and a level 40 Carbink.
Go up the stairs to the right of Hiker Alain.
Frost Cavern 2F, part 2
Turn on your Dowsing Machine to find a hidden Super Potion on the rock to the right.
Go north and up the stairs.
Frost Cavern 3F
Slide north. Black Belt Kenji is running around. He has a level 46 Gurdurr.
If you go slightly north and then slide east along the top edge of the ice, you can reach Hiker Brent to the east. He has a level 44 Probopass. But there is nothing else of interest where the Hiker is, so you can ignore that area if you want.
On the area where Kenji is running around, there is a rock on the right side. Walk left from that rock until you are as far as you can go without sliding. (If you do slide, just slide back.) Then slide south from there. You should bump into a rock in the ice. Slide west, then south, then east, then north and you will reach an Ether.
From there, slide east, then south, then west, then south, then walk east. You should be back where you started. Slide north to where the Black Belt is. There is a rock on the right side. Walk left from that rock, then slide south, and then east. When you reach the wall, slide south, then west, then south.
To the south, you can avoid a battle by sliding over the narrow strip of ice to the right of the Battle Girl. If you don't avoid her, you encounter Battle Girl Gabrielle, who has a level 46 Medicham.
Use your Dowsing Machine to find a hidden Elixir on the rock to the right of the Battle Girl.
Now slide east on the ice. Go down to the south path and slide south. Slide across to the narrow eastern path to find a Zinc.
Slide west, then go south and down the stairs.
Frost Cavern 2F, part 3
Slide east along the top edge of the ice, then south. When you are in the southeast corner, slide diagonally northwest. When you hit the wall, slide south to reach TM79 Frost Breath.
Slide north, then west, then go up the stairs.
Frost Cavern 3F, part 2
Go north and slide north on the ice, then slide west. Slide north, and then west. From there, slide south, then east, then north, then east, then south, then west, south, and finally walk west to get back to where you started on the third floor.
Slide north and go past the Black Belt. Keep going north and you'll find Team Flare.
Battle Team Flare
Trevor will run up. You'll notice Team Flare preparing to catch an Abomasnow because it emits a lot of energy and makes more snow fall the more threatened they make it feel. Trevor wonders why they want to gather pokémon, energy, and money, and the blue-haired scientist says that it's so only Team Flare can survive.
Now you have to battle a Team Flare Grunt who sends out a level 42 Golbat followed by a level 42 Manectric.
After the battle with the Grunt, turn on your dowsing machine to find a hidden PP Up to the right of the square-shaped pit on the right side of the room.
Now talk to the blue-haired Team Flare scientist. This is Mable. She has a level 48 Houndoom.
Receive a Mega Stone
After you defeat Mable, Team Flare leaves. Talk to the Abomasnow to get Abomasite. This is the Mega Stone that will allow you to Mega Evolve an Abomasnow.
After that, you can leave the cave. You should now go back to Dendemille Town and then use the southeast exit to reach Route 17: Mamoswine Road.