Mistralton City is, one could say, an airport with a town attached; it's quite small, home to few services, but you will have to challenge a Gym here.
In the house east of the Pokemon Center is the Move Family, the old man is the Move Deleter who can make your Pokemon forget a move, even an HM move, and the girl is the Move Reminder who can let them learn an old move they forgot in exchange for a Heart Scale (these are quite rare, but can be found in Driftveil City).
As you go north into town, a man walks up to you and introduces himself as Cedric Juniper, the father of Prof. Juniper! He will upgrade your Pokedex with new forms to show more information. He will tell you about Mistralton City's Gym leader, Skyla, how is out tending to a sick Pokemon at the Celestial Tower, on Route 7.
So, head north onto Route 7, with that!
- Watchog (Lv. 27/31-33 in dark grass)
- Deerling (Lv. 26/30 in dark grass, dep. on season)
- Zebstrika (Lv. 27-29/31-33 in dark grass)
- Foongus (Lv. 29/31 in dark grass)
- Tranquill (Lv. 26-28/30-32 in dark grass, not in winter)
- Cubchoo (Lv. 26-28/30-32 in dark grass, winter only)
- Audino (Lv. 26-29)
- Unfezant (Lv. 29)
- Emolga (Lv. 27-29)
- Emolga (Lv. 30, trade for Boldore)
Route 7 is notable for its very tall grass, as well as a house that lets you trade a Boldore for an Emolga. Use the northern path to Celestial Tower. This is also your second chance to try the new battle styles, with Ace Trainer Elmer, who offers a Rotation Battle in Black, or a Triple Battle in White.
- Ace Trainer Elmer (Rotation or Triple battle)
Similarly to Pokemon Tower in Kanto, Mt. Pyre in Hoenn or Lost Tower in Sinnoh, Celestial Tower was built in memory of dead Pokemon. There are a number of trainers throughout the site, and you need to progress to the roof to meet Skyla.
Once you reach the top, Skyla is standing there, having helped the sick Pokemon. She lets you ring the bell, and then invites you to a Gym battle back in Mistralton. Therefore, exit the tower, and return to Mistralton City.
The Gym is located in the large building, north of the runway (which isn't actually the airport terminal). It specialises in Flying-type Pokemon, so bring along Rock-, Electric-, and Ice-type moves to deal super-effective damage (Electric is recommended due to Water/Flying-type Swanna).
This Gym contains easily the most dangerous structure in any Pokemon game; one must be shot out of cannons pointed correctly to reach Trainers and eventually Skyla.
- Leader Skyla
Skyla's gym lineup, while perhaps not quite as intimidating as Elesa's or Clay's, can be a serious pain to take down. All three have moves with a chance to flinch (Heart Stamp for Swoobat and Air Slash for Unfezant and Swanna), and Unfezant also has Quick Attack. Swoobat knows Acrobatics, which is quite powerful. If you have an electric mon like Zebstrika or Emolga, this battle will end quickly, with Swanna's 4x weakness to electricity.
After defeating Skyla, she'll give you the Jet Badge, plus TM62, containing Acrobatics, a move which doubles in power with no held item.
Outside the gym, N is waiting for you, and will tell you about Ghetsis's master plan to control the legendary dragons of Unova.
Now you have six gym badges, the next stop is Icirrus City, which can be reached through Twist Mountain. So, going towards the mountain, head north onto Route 7!