I was warned about Fantina, who completely stopped my friend in her tracks, but I didn't have much trouble with her. This may have been because I paused my progress to train up a rather nicely-IV'd Gyarados with Bite; I wasn't fast enough to outrun Mismagius of course, but it was very helpful. I actually had the most trouble with Volkner, and it was probably a good thing since at that point in the game I get rather sick of battling Gym Leaders and I just want to move on to the Frontier where I'll actually be challenged. If I had had something other than Torterra with Earthquake I might have had an easier time, but as it was that was the gym where I used the highest number of items. I've not had to soft-reset at any gyms in the game.