For when a Nuzlocke is just too easy
The Hardcore Nuzlocke is a variant of the regular Nuzlocke challenge that is designed to be more difficult and encourage strategic play by preventing players from using common tactics such as over-levelling team members or relying on healing items during battle.
Normal Nuzlocke rules apply, with two primary additions:
The player may not use items in battle. Pokémon may hold items, but no items from the bag (except for Balls) may be used.
The player may not level any of their Pokémon past the level of the next “boss trainer’s” highest leveled Pokémon. “Boss trainers” can include Gym Leaders, Kahunas, team admins/leaders, and, when entering the Pokémon League, the final Elite Four member. Any Pokémon who exceed the level cap must be boxed, and cannot be used until they are under the level cap. Exceeding the level cap during the battle is allowed, you must simply enter the battle with no Pokémon over the level cap.
Additionally, while not one of the two main rules, it is generally accepted that Set mode should be used for the Hardcore Nuzlocke.
here's an example...
A great summary from Pokemon Challenges of how Hardcore Nuzlocke's work - enjoy!