Personnaly, I found Shelly to be really hard.
My team were mostly special attackers and the struggle bug spam was really annoying. Also, I had no pokemon with Flame Burst, so that played a major role.
Even with that, (Growlight up to level 28 for Flame Burst), her Anorith really annoyed me, with surprisingly high speed and high attack rock slide. Also with Aqua Jet against my Golem ><
Easiest for me was Serra. I had a Delphox, but the most useful was Magneton with Flash Cannon. Never missing, always super effective. Beat her on the first try. Same for Julia and Florinia, beaten on the first try, but Serra was really easy.
Kiki was fairly easy, only thing that made me restart 2-3 times was that Medicham that dealt more damage that I thought (Damn you Pure Power), one shotting my Delphox with Strenght after one Meditation.