I don't know what a nuzlocke is; I was just doing some leveling up for him while he was away.
The Nuzlocke Challenge is a set of rules for a more difficult playthrough experience, there are many variants and the rules are completely up to the player but the basic ones are generally
1. If a Pokemon faints, they are dead and can no longer be used
2. You can only catch the first Pokemon you see when you enter a new area, nothing else