Other than maybe Roxanne and Roark, Sinnoh and Hoenn Gym leaders fail in general. I found that as long as your pokemon match their levels, you'll have no problem at all. In contrast, Kanto and Johto gym leaders were a lot tougher, had better AI, and had strong countermeasures against their type weaknesses, so matching their levels wasn't enough, you had to have countermeasures to their countermeasures. :P