For me, a tie between Lenora and Drayden.
I was a little unprepared when I faced Lenora, I caught myself a female Timburr (yes, FEMALE) that did most of the work (I had Dewott as a backup) but oh man, Retaliate really screwed me. I won on the first try though.
Drayden was just as hard as I didn't have any Ice-types on my team. His Dragon Dance spamming literally got me scared, I beat him in the end though with just two Pokemon.