After you have defeated Dark Rust in the final part of the story, when you return to Easterly Town, a Rattata will inform you of a new mode; The Battle Locale. The task you have here is to complete it with none of your reserve toys at all.
When you enter the mode, you will get sent to a random location from the game and have a Rattata placed in front of you. This Rattata is of a level lower than the average you'll find in the level. Once you enter the level, you will be able to collect other Pokémon and use them in your attempt to defeat the level. However, as the levels are usually themed with certain types, this can be quite a challenge.
If you manage to successfully complete a level, you will get a prize in the form on money. However, should you have succeeded in a streak, the prize is increased. The prizes are as follows;