Hmm... I think this generation was challanging to me. Well, I think my team had many flaws, but also I was rushing to get my national dex ASAP. I tried to complete at least one gym a day.
Also I foolishly challanged the Elite four with pokemon in the low 50's. I barely won, but I had to restart so many times and wasted a gazillion revives. The most trouble I had was with Lucian and Cythina, since with Lucian, my Luxray had horrible sp.Def, and kept on being killed by Alakazam. Cynthia's Lucario gave me trouble, as I didn't have a trained flying pokemon.
I think this generation was much difficult than the 3rd generation, in which the Elite Four was really easy. I think it might've been that swamperts are very useful.