To be honest, I almost died in all of them :[
Maylene was a big challenge for me, because my team was pretty under-leveled and my team is mostly dark types, (before with my Eevee being a normal type, which evolved into Umbreon right after I beat Maylene!) Crasher Wake's Gyarados was unusually tough for me, but I managed to beat it w/ my Umbreon's Confuse Ray and Toxic combo.
Gardenia was probably the easiest; breezed through her team w/ my Gible's swords-danced boosted fire fang. Her Roserade managed to KO my little Gible though. Murkrow managed to win w/ Drill Peck. Fantina's battle wasn't hard, but it wasn't easy. It was very fun taking on a team of Ghosts with my Dark types. Roark was surprisingly hard for me. I had all of my team averaged at Lv 6. Cranidos is a powerhouse; his Cranidos killed mine. |D I got lucky with Earthquake and SuperPower and won.
All of the gym battles were very enjoyable to me. I can't wait to rebattle them after the game. <33