And in my recent Red Mono-Color on White, i'd say the hardest Gym was between Cress, Elesa and Skyla. The former because I only had Tepig (I had to level up to 15 to get Flame Charge, and even then it still took three tries after that). Elesa because over half my team was weak to Flying, and in the end it came down to my Sandile with less than half its health being my last Pokémon and I got Zebstrika to -3 Attack, and I was using Rest throughout the match. It was my last Pokémon, but I still won first try. And Skyla because, again, my team was horribly weak to Flying and I really had to strategize.