I have beaten all the current released gyms, I will admit they are very difficult at times, but I find it a lot more enjoyable to actually have to use my brain and strategize how to defeat the gym leader rather than simply being able to fly through like they are nothing, in Reborn I can enjoy a difficult and fulfilling battle with a gym leader in which our levels are close and it actually feels like I achieved something by beating them. Pokémon Reborn isn't easy and if you aren't willing to train pokemon you usually wouldn't you will find it quite difficult without overleveling, although as already stated you can use Common Candy to lower the level of your pokemon so that then bring it back to obedient level.