Easiest for me: Julia (1), Corey(1), Shade(1), Cal(1), Kiki(1), Aya(2), Radomus(1), Luna(1), Samson(2), Terra(1).
Harder ones: Florinia(4), Serra(6), Noel(7),
Hardest: Shelly(12) and Charlotte(13) Blake(15)
The numbers behind it are the amount of times it took me to beat them. I find gym battles more fun to train than grinding, you never know when you accidentally win.