The pokémon that you can catch here in Celestial Tower are Litwick, which is a Ghost/Fire pokémon, and Elgyem, which is a Psychic type pokémon that is based on extraterrestrials, also known as little green men, or LGM (Elgyem) for short.
There are a lot of trainers here in Celestial Tower, so you might want to take some time to think about the party that you want to bring. You should definitely bring pokémon that know Ghost moves and Dark moves. It would also be good to bring pokémon that know Ice moves and Rock moves. I also recommend that you have at least one pokémon that knows Fighting moves, and finally, a pokémon that knows Fire or Flying moves and Electric or Grass moves.
An example party that might work well is Yamask, Liepard, Vanillite, Boldore, Emolga and Pansage.
Going up the tower
On the first floor of the tower, there are no wild pokémon or trainers, so go on upstairs.
On the second floor, you will encounter Psychic Doreen, who has an L30 Elgyem, Solosis and Sigilyph. For Elgyem and Solosis, use Dark, Ghost, or Bug moves, and for Sigilyph, use Electric, Ice, Rock, Ghost, or Dark moves.
Next up is Lass Kara, who has an L30 Deerling and Gothita. After that is PokéFan Jude, with an L30 Emolga and Herdier. Finally on this floor is PokéFan Georgia with an L31 Swadloon.
Be sure to pick up the Hyper Potion and TM61 Will-O-Wisp on this floor.
On the third floor you find Psychic Belle with an L32 Musharna, Psychic Micki with an L31 Yamask and Golett, Psychic Lin with an L31 Litwick and Swoobat, Psychic Bryce with an L32 Duosion and finally Nurse Sachiko with an L31 Gothita and Alomomola. When you defeat Nurse Sachiko, she will heal up your pokémon for free.
Be sure to pick up the Spell Tag on this floor.
Finally, on the fourth floor, you encounter Ace Trainer Beckett with an L33 Stoutland and Ace Trainer Kassandra with an L32 Swoobat and Gothorita.
Be sure to pick up the Revive and TM65 Shadow Claw on this floor.
Now you can go to the top of the tower, where Skyla says that she gave the injured pokémon a Max Revive so that it could fly away. She asks you ring the Celestial Tower bell to soothe the spirits here, so do that. Then Skyla will say to challenge her at Mistralton Gym.