In Mistralton City, Skyla recommends that you train in Celestial Tower. To get there, go northeast from Mistralton City and along Route 7.
Celestial Tower 1F
Talk to Professor Juniper to get a Lucky Egg, which increases earned EXP by 50% when a pokémon holds it. Go upstairs.
Celestial Tower 2F
Near the beginning of the second floor, you find Psychic Joyce. She has a level 36 Musharna. To her left, there is a Hyper Potion. You have to go right, then up, then left to get it.
To the right, there is School Kid Alberta, who sends out a level 35 Litwick. To her right is TM61 Will-O-Wisp.
Up above, there is Pokéfan Jude. He has a level 35 Clefairy.
Celestial Tower 3F
Just south of the stairs is Pokéfan Georgia. She has a level 35 Cubchoo.
Below her, there is Psychic Micki. He has a level 36 Espeon. To his left is a Revive.
To the right, there is Psychic Bryce. He has a level 35 Elgyem and a level 35 Duosion.
South of him, there is Nurse Dixie. If you defeat her level 35 Leavanny, the nurse will heal up your Pokémon.
Celestial Tower 4F
To the left, there is Socialite Grace who has a level 35 Roselia. To the left of her, there is a PP Up.
Up above, there is Gentleman Daniel with a level 35 Umbreon.
To the right, there is Psychic Sarah, who sends out a level 35 Yamask and a level 35 Gothorita. Near her is TM65 Shadow Claw.
Celestial Tower 5F
At the top level of the tower, use your Dowsing Machine to find a Ghost Gem to the left of the bell platform. That's all you can do in the tower, so go back to Mistralton City to challenge the Mistralton Gym.