As a lot of people here have said, Pokémon is supposed to be a children's franchise and has always been easy, but the games do tend to throw Pokémon that can really mess with you on occasion.
As i've said before, I don't think XY are that easy without the Exp. Share, because of the gigantic variety that trainers can throw at you and how I always tend to feel under-leveled on occasion. Sometimes after the last Gym i'd be around 50-52.
For that matter, there are many people who call BW the easiest games in the series, but I will never say that. Gym Leaders like Lenora, Elesa, and Clay can be known to be notoriously difficult considering how few options there are. The Elite Four I don't think are that easy either for that reason. N uses a legendary for goodness sakes, and a fast Rock/Flying-type, and don't even get me started on Ghetsis x_x Although to be fair, most of my BW plays have had something to ready me for that Hydreigon (whether it be a Fighting-type, Dragon Dance Haxorus or such).
Some people say that BW2's Challenge Mode was a waste even without the arbitrary requirements, but for what it is, I like it. It can leave you under-leveled against the trainers later in the game, which, after watching Drayano's Poison Monotype and seeing how lower leveled he was than the Gym Leaders, i've tended to let that happen in most of my challenges recently (BW2 or otherwise), except Nuzlockes and Wedlockes. It does make things more interesting.