Laying stealth rock is pretty tricky because her two lead pokemons have energy ball and bubble beam , the Yanmega also has giga drain, Shelly's forest field makes it worse. I still can lay stealth rock but this also mean sacrifice Onix in next turn.
I catched a Klink in Railnet under the emty housre near pokemon center in Obsidia Ward. The power plant is closed so I can't get Magnemite. I Will train the Klink and evolve Prinplup, hope this is enough to beat her.
Thank you all.
p/s: I'm a chick.
EDIT: I win. I sacrificed Onix for set up stealth rock, Shelly keep using attacks have low stats effect so Empoleon, with his ability defiant, got huge attack buff, he easy sweeps all her team.
Her team no longer had Scyther. Now she has Masquerain, Volbeat, Illumise, Anorith, Yanmega and Wormadam (trash form)