I literally just beat the elite 4 and trained 4 of my Pokémon to level 90 to do it. I tried it with 4 of them at 80 and 2 at 70, but I couldn’t get past Lance’s Dragonite at that level since I didn’t have any ice, dragon, or fairy type moves to fight it with. Now that I’m moving on to other regions I hope gyms are still a challenge even given how high level my Pokémon are.