The Aspertia City Gym is straightforward. It's a small building with a few trainers. The 'puzzle' involves navigating a few platforms. Simply walk forward and battle the two Gym Trainers before reaching Cheren. These trainers will use Normal-type Pokémon like Patrat and Lillipup. Use this opportunity to gain some extra experience and warm up for the Gym Leader battle. Remember to heal your Pokémon at the Pokémon Center if needed before challenging Cheren.
Recommended Pokémon: Fighting-type Pokémon are super effective against Normal-types. If you caught a Riolu on Floccesy Ranch, it will be incredibly useful here. Otherwise, any Pokémon with strong attacking moves will suffice. Even your starter can do well if it has a type-neutral attack or if you've leveled it up sufficiently.