What do you think are the most difficult boss fights in the series? Could be a rival fight, a Gym Leader fight, basically any major battle.

I think pic related is a killer, at least on Challenge Mode. Her Emolga beats your starter (Volt Switch against Dewott, Aerial Ace against Servine and Pignite), and even when I tried to hard wall it with Magnemite, she just switched to Zebstrika which could have almost wiped the floor with my team if it wasn't for me getting lucky with Whimsicott stalling it out.
It was fun though.