While agree with most of the answer, and would say level 50 is fine in Normal Gameplay, but it seems a bit too low for a Nuzlocke in my opinion, especially when they have the Elite Four to face, as well. If they grinded just 6 Pokémon to level 50, they be just below the Pokémon of the Elite Four. In addition, one member fainting beforehand could be a crucial downfall against Cynthia, with her Pokémon ranging from Levels 60-66. Like I said I agree with most of the answer but I think it will be better to grind to Level 55-60, depending how prepared you want to be for the fight. While you could get lucky at level 50 and sweep, chances are a few Pokémon may go down In the process even before reaching Cynthia.