Instructions
Walk to Route 111 outside of Mauville City, a small path populated by a family of campers.
Speak with the matriarch of the family, Victoria, just to the right of the entrance to the area. This begins a battle with her family members.
Face the duo of Gabby and Ty, the picnicker Irene and Camper Travis back-to-back without stopping. These trainers have five Pokémon between them ranging between levels 18 and 19. All of the Pokémon are susceptible to fire-type Pokémon. Since players have six Pokémon to choose from at any given time, these battles don't present much difficulty.
Speak with Victoria after defeating her family to receive the Macho Brace.