Here's how to find it:
1. Enter Aspertia City.
2. Head towards the Pokémon Center.
3. You'll see a sign on your left that says "Rotation Battle!"
4. Approach the sign to start the battle.
The rotation battle is a great way to learn the ropes of this battle style. You'll face two trainers who use a rotation system, switching their Pokémon out to exploit type matchups.
Remember that in a rotation battle, you'll have two Pokémon on your team at once, and you'll be able to switch between them during the battle.