The Unova Challenge can be found in Black City's Black Tower or White Forest's White Treehollow . To access the challenge, you have to defeat Alder in his house in Flocessy Town . In the challenge, your aim is to progress through ten randomly-generated areas occupied by Pokemon trainers.
In each area, there is a Gate Trainer and Boss Trainer. First you must defeat the Gate Trainer to unlock the gate to the Boss Trainer, then defeat the Boss Trainer to complete the area.
There are a few complications however:
You can't distinguish who the Gate Trainer is. Should you encounter a trainer who isn't the Gate Trainer, they may give you a hint to locate the Gate Trainer.
You cannot use items at all. If you need to heal your Pokemon, you can seek a Doctor or Nurse. Of course you will need to defeat them first and they only heal you once.
Some rooms are pitch-black, so a Pokemon with Flash is advised. Since you can't use items, you can't teach Flash on the go, so be aware of that.
Unlike other battle facilities, you can earn experience and money in battles here. To accomodate this, the opponents' Pokemon will Level Up the higher up you go.
If you need to leave the tower or treehollow, you can use the 1st Elevator by the entrance you came from or open the menu and pick the " Retire " option. Additionally, Areas 6 and beyond have multiple floors, which you access via the Elevator used to enter and exit.
Once you've cleared Area 5, you will receive the Tower Key if playing Black 2 or Treehollow Key if playing White 2. These keys unlock Black City and White Forest respectively. The key is useless for you, but can be sent to players of the other version.
Upon clearing Area 10, you will receive a Shiny Gible (Black 2) or Shiny Dratini (White 2) should you return to Alder's house. Score!