Since I am a lover of using underrated Pokemon, I found it extremely hard to beat Cynthia my first try. If I remember correctly, I had a a Vespiquen, Cherrim, Lumineon, Magmar, Pachirisu and I think Chimecho?
Basically I had to keep grinding until Lumineon could Ice Beam her Garchomp. Most of her other team members were fairly straight forward. Gastrodon was taken down by Cherrim, Magmar took care of Roserade, Milotic was Vespiquen and Cherrim, and Lucario was just whoever was alive at the time lmao. Meanwhile Spiritomb was stalled out via toxic and full restores. :(
Good times.