With the Mayor's escape, Rui suggests heading to Pyrite Town, a lawless settlement known for its shady dealings. This will be your next destination. Before leaving Phenac City, ensure your Pokémon are healed and you have a good supply of Poké Balls, Great Balls, and Potions. The journey to Pyrite Town involves traversing the desert, and you'll encounter a few trainers along the way. These battles are good opportunities to gain experience for your Espeon and Umbreon, and any newly snagged Pokémon.
The path to Pyrite Town is linear. You'll encounter a few trainers who are not part of Team Cipher, but still offer good experience. Use this time to get comfortable with double battles and managing your team's health. Remember to save frequently, especially before entering new areas or engaging in significant battles.