Cynthia in white 2.
This is not because it is hard, it really is not, just the tactics that she uses. Because there really is nothing to do from levels 66-74 in the game, I've just been challenging Cynthia and since you are under-leveled and can't O/2HKO her pokemon you realize how annoying some of her pokemon are. (Also come on, she must have a high crit rate)
Spiritomb: Use a flipping deerling or stantler or something :P But besides that, double flipping team all the time and pressure just makes you run out of pp so much.
Milotic: FUCK OFF MILOTIC! I try to play pokemon in my spare time, but milotic just rest spammed in about 4 of my spare times! All I had left at the point was my keldeo, aggron and camerupt so obviously I had to use keldeo. Keldeo was just critting its hydro pumps left and right, about 4 times against milotic, but it never did it twice in a row, and milotic would just use rest again... and again.. and again.... (and then carry on for, like, 13 times). I counted, she uses rest 13 times on my game before she runs out. Effing cheater...