From Valor Lakefront, head east onto Route 222. This is a coastal route with many trainers and wild Pokémon like Electabuzz and Luxio. Battle trainers and collect items. You'll eventually reach Sunyshore City, a vibrant city powered by solar energy.
Heal your Pokémon. You'll notice the Gym Leader, Volkner, isn't in his Gym. Talk to Flint, an Elite Four member, who will tell you Volkner is at the Lighthouse. Head to the Lighthouse in the southeast of the city and talk to Volkner. He'll return to his Gym, ready for your challenge.