Thats exactly what i did, then use nummel to flame burst wich worked, but her illumise wont go down with one hit no matter what i use, if i use my spooky owl, my nummel doesnt matter she always survies, the one time i did crit her, she just summoned the spider type pokemon she has (not good with names of newer gen pokemon) and cast rain with that
When i last did this game i do not remember shelly being this hard, it was the second gym that killed me, but that was a breeze this time