I didn't really find any of the Gyms that difficult, maybe because I have a tendency to train like crazy right before a challenge and then find out that my levels are miles above the Gym Leader's.
On the Elite 4, I didn't really have any problems either. Aaron was probably the trickiest, but I got in a critical-hit Earthquake on his Drapion and another critical Aerial Ace on his Heracross. Bertha was incredibly easy to beat with Empoleon and Leafeon's Grass Knot, and I got about four more critical hits on Flint with Surf. Four of Lucian's team died to Weavile's Night Slash, and I got in another critical hit Aerial Ace on Gallade and OHKO'ed it. o.o Even Cynthia's Garchomp went down in two hits (Ice Beam + Earthquake.) The only Pokemon in the League that really gave me trouble, ironically, was her Togekiss, who knocked Luxray out four times in a row with Aura Sphere because Thunder Fang kept missing.
So I guess it was a mix of overpreparedness and sheer luck.
That was a really long post. x_X