In Mistralton City, Skyla suggested going to the Celestial Tower for training. To get there, first go northeast from Mistralton to Route 7.
Explore Route 7
There is a PKMN Breeder to the left who will battle you whenever you enter the area. To the north, a Youngster will do a Rotation Battle with you. North of him is a Zinc and a pokéball that is actually a Foongus.
Talk to the Hiker in the house and give him an Emolga to get a level 35 Gigalith. You can get Emolga in shaking grass in this route.
Check the trees to the right of the Hiker's house to find a Hidden Grotto.
East of the Hiker's house is a Lass who will do a Triple Battle with two level 33 Duckletts on the outside and a level 33 Unfezant in the middle.
North of the triple battle Lass is a Nursery Aide with a level 34 Cleffa and a level 34 Cinccino. Farther north there is a Preschooler with a level 34 Sunflora.
Go west of the Preschooler to find a Leaf Stone in the grass. There is a fake pokéball sitting out in the open to the north that is actually a Foongus.
If you keep going north, you'll reach Celestial Tower, but you can get a TM if you take a detour to the east. First, go east from the Nursery Aide to find a house. Talk to the old man to see pictures of Tornadus, Thundurus, and Landorus, which will add them to your Pokédex. Use your Dowsing Machine outside of the house to find a hidden Revive.
If you continue east, your Mom calls to check up on you. The nearby twins will do a double battle with you, using a level 33 Zangoose and a level 33 Seviper.
East of the twins, a Harlequin battles you with a level 35 Baltoy. Near him is TM81 X-Scissor. North of him, there is another Harlequin with a level 35 Baltoy.
You can't go any farther to Twist Mountain because Marshal is blocking the way. Go back west, past the house, then north. There is a Preschooler hiding in the grass to the east who has a level 34 Darumaka. Behind her is an Elixir.
Continue north to Celestial Tower.