At this point in the game, we have strong pokemon, so the only advantage gym leaders have is their interactions with the field effects. So, the way to win is to take that away. For example, I've got a bulky Tangrowth with Grassy Terrain. This destroyed Amaria, because when she loses her speed and damage boosts, shes just a bunch of water types that get wrecked by lightning or grass. So try and find some mons that can change field effects