Walkthrough
After defeating Marnie in battle, she will show you the secret entrance to Spikemuth. This small town is where Marnie was born and where Team Yell resides. There is not much to this town outside of the Pokemon Gym, the same that doesn't have access to Dynamax Pokemon since where it is located in The Galar Region. You will basically be thrown in the Gym Mission when you enter the city simply by heading to the east, so make sure to heal your Pokemon at the Pokemon Center right when you gain access to Spikemuth. (After defeating the Main Story, you can return to Spikemuth to face Marnie in a battle as a Daily Event.)
Trainers
Pokemon Trainer Marnie: Level 59 Liepard, Level 59 Toxicroak, Level 59 Scrafty, Level 60 Morpeko, Level 60 Grimmsnarl
Items
Max Revive: In Middle Alleyway
Choice Specs: South Of Piers' Stage
Pokemon Center PokeMart Items (Through Storyline Progression)
Poke Ball, Great Ball, Ultra Ball
Potion, Super Potion, Hyper Potion, Max Potion, Full Restore
Antidote, Burn Heal, Ice Heal, Awakening, Paralyze Heal, Full Heal, Revive
Escape Rope, Repel, Super Repel, Max Repel, Poke Doll