Black Skyscraper/White Tree Hollow

Before you can enter the challenge, you will have to beat Alder in Sanji/Floccesy Town..

Here, there's a few basics that you need to know..

• There's no restriction to which Poke you use.. Means you can use Legends here..
• There's no lvl restriction.. Means you can go with Lvl100 Pokes..
• You can use up to 6 Pokes.. Again, no restriction in number of party..
• You can't use your Bag AT ALL.. Not even being able to teach your Pokes TM there..
• You can switch your Poke when your opponent's Poke knocked out..
• You can save ANYTIME inside the Area.. 
• Pokemon that seen here will not be recorded in Pokedex..

Knowing that, I would highly suggest that you have at least 3 Pokes lvl 8x here.. Also, get your useful items like Leftover attached to them before entering the Area..

There are 10 stages.. Each stages are called Area 1, Area 2, etc... You need to win Area 1 to proceed to Area 2 and so on.. So of course the difficulty and their Pokes' average lvls increase after each Area..  Area 1 to 5 are pretty simple.. Their average lvls are 5x~6x.. Not only that, you will just be playing around 1 floor each.. For Area 6 to 9, you will be playing around 2 to 3 floors.. And their lvls are about 6x~7x..

Each floors will have few rooms, and each rooms will have 0 to 3 trainers.. All trainers will only use 2~3 Pokes..You have 2 missions.. First is to find Gate Trainer, who opens the gate.. And the second mission is to find the Gate that will be opened.. The Gate and the Gate Trainer doesn't have to be on the same floor (For Area 6 to 10).. Once the Gate is opened, you will have to fight the Boss Trainer, who is the strongest in the Area.. His lvl will be slightly higher than the others..

Each time you defeat a trainer, except the nurse, MOST of them will give you hints.. It should be one of these..

• The gender of Gate Trainer.
• The number of people in the same room with Gate Trainer.
• The Gate Trainer is stationary or moving.
• The nurse's location. 
• The Gate Trainer is on the above/below/"this" floor. (You will receive this information from Gate Trainer)
• The first Poke the Boss Trainer will use..

Unlike Battle Subway, you don't heal after each single battle.. However there will be a nurse each floors.. Like the other nurses, you will have to defeat him first.. BUT one nurse will only heal you once.. Good thing is that you can choose when to heal.. You can reject his first offer after his defeat and come back to him again to heal your Pokes.. So I would highly suggest that you defeat the nurse the moment you see him.. Use the nurse wisely because he's the only way you can cure your Toxic-ed or Thunder Waved Pokes here..

Area 10
Here you will be playing around 4 floors.. The Boss Trainer is Banjirou, Alder's grandson.. He will be using Latias, Garchomp and Volcarona.. All his Pokes are at lvl 80.. After you have defeat him, go to Alder's house in Sanji Town to claim your reward.. Talk to Banjirou there and you will receive a shiny Gible/Dratini..

Market
There's actually 4 shops in Black City/White Forest.. Each shop will open when you beat a certain stage.. The shop will only sell one item each day, and it's fixed..

Shop 1: Sells item for 1000 dollars
- Black City:  Thunderstone or Protector
- White Forest: Leaf Stone or DeepSeaTooth

Shop 2: Sells item for 2000 dollars
- Black City: Fire Stone or Metal Coat
- White Forest: Water Stone or  DeepSeaScale

Shop 3: Sells item for 4000 dollars
- Black City: Dusk Stone or Up-Grade
- White Forest: Dragon Scale or Shiny Stone

Shop 4: Sells item for 6000 dollars
- Black City: Dawn Stone or Dubious Disc
- White Forest: Oval Stone or King's Rock