The Nuzlocke Challenge as it is called is a means of making any Pokemon game more difficult, including Black 2. Basic rules are
- You may only catch the first Pokemon you see in each area (route city forest etc.). You can not catch any other Pokemon from this area thereafter. If you do catch another Pokemon from an area, you can't use it. If the first Pokemon flees or faints, you can't try again.
- If the first wild Pokemon battle is a double battle, you may catch either Pokemon.
- If one of your Pokemon faints, you can not ever use it again.
- If all of your party's Pokemon faint, its game over
There are many other additional rules one may employ, listed here.
Hope this helps