While Elesa's duo of Emolga proved difficult to nearly every team I used, I think that Lenora is by far the hardest. Her Herdier is pretty bulky and hits hard, so even that proves a formidable foe, but her Watchog is by far the hardest Pokemon to take down, in all of Unova. Once you finish off Herdier, you're looking at a 140 BP STAB Retaliate, which OHKOs pretty much every Pokemon you can get at that point, save Roggenrola. Crunch and Retaliate without the boost also tend to hit pretty hard, both having extremely high base powers for a Pokemon in only the Second Gym. Hypnosis is probably the main reason why this thing is annoying, if you don't have any Awakenings, then your chances of winning are slim to none, as you're effectively taking attacks which probably KO your Pokemon in 2-3 hits, while trying to wake up. Watchog too strongth x)
Ironically enough, though, Lenora is also my favorite Unova Gym Leader.