Recommended Team: Your most kick ass Pokemon!
The first thing to do upon returning to Aspertia City is to heal your Pokemon at the Pokemon Center if you need to. Once you're ready, enter the Trainers' School located to the left of the Pokemon Center.
Inside the Trainers' School you'll find a number of people. Talking to the person north of the top-right desk in the main room of the school will earn you a X Defend . Other than that, you can check out the blackboard for information on a variety of battle-related things. Once you're done, exit the school through the north exit.
You're now in the first gym of Pokemon B/W 2! What's that? 'But we're outside!'? Well, for the first time in a Pokemon game, a Pokemon Gym is outdoors! The gym leader himself is none other than Cheren, one of your old rivals from Pokemon B/W.
As soon as you enter the 'gym', a man named Clyde will approach you and give you Fresh Water , as well as explaining what's special about this gym. When you're done talking to him, you will catch Cheren's attention. After talking to Cheren, you now need to defeat the two trainers waiting for you.
You can't simply skip past them and battle Cheren, but must battle them both before being given the chance to battle the Gym Leader. You can battle them in either order, although it's assumed that you'll battle the Youngster before the Lass. When you're ready, talk to the Youngster to begin the first battle.
Gym Member Battle: Youngster
Once the Youngster has been defeated, head back to the Pokemon Center and heal your Pokemon if you need, then begin the battle against Lass Serena.
Gym Member Battle: Lass
Once the Lass has also been defeated, it's time to battle Cheren! Heal your Pokemon at the Pokemon Center first if you need to (it's recommended, as you're going to need a healthy team to take him on) and talk to Cheren to begin your battle!
Gym Leader Battle - Cheren
Note : Don't forget to save before you enter the battle! This means that if you lose the battle and don't want to lose money, you can simply restart and try again.
As both of Cheren's Pokemon are Normal-type, none of the Pokemon that you could have caught thus far will have any type advantage or disadvantage, except for Riolu who has a type advantage against Normal-type Pokemon being a Fighting-type.
If you have Riolu and you're struggling to defeat Cheren, it may be worth training it to Lv.15 so that it can learn Force Palm. With this move it can easily single-handedly defeat Cheren's entire party. Besides this, Riolu has Counter, which returns double the damage that Riolu takes by any of your opponent's physical moves.
Even without Riolu and its Force Palm, this battle isn't extremely difficult, assuming that you have a full party that average about Lv.10 or higher. Simply use the moves that have the highest power, factoring in whether a Pokemon gets a power bonus by using a move of its type. Grind through both of his Pokemon until you've won.
A big thing to watch out for though is Cheren using a Potion on either of his Pokemon, as this can mean that you're practically having to defeat it twice. Cheren will only use the Potion if one of his Pokemon becomes low on HP. Keep an eye on his Pokemon's HP and don't hesitate to use weaker moves on occasions.
Once Cheren's defeated he will reward you with the Basic Badge! Good work! Go reward yourself by fist-bumping a random or something! You'll also receive TM83, which teaches Work Up , as a secondary reward. Once Cheren finishes speaking, it's time to leave the gym.
Outside you'll be approached once more by Bianca. She'll give you TM27, which teaches Return . Cheren will then meet you outside of the gym and have a lovely little reunion with Bianca. Bianca will then give you the Xtranceiver and a four-way call will begin between you, Bianca, Cheren and Professor Juniper.
Once the call and the conversation ends, your rival will appear. After yet more conversing, Cheren and Hugh will leave and Bianca will give you the C-Gear . With this, you can now trade with your friends! After this, you'll finally be free to go. Heal yourself and leave Aspertia City. Onwards to Virbank City!
Continue to Floccesy Town and exit to Route 20. Just before you do, Bianca will (once again ) stop you and talk to you. Answer 'No' to her question this time. She will eventually leave and you can now continue until you're back in Route 20.