Volkner was the hardest for me. I usually pick Chimchar at the beginning, but this time I wanted to go with Turtwig and see what I could do. Everything fine until I fought Volkner. I had no idea his Pokémon had Ice-type moves, so he caught my Torterra off guard. If you're going to fight him, make your one of your Pokémon isn't weak to Electric- or Ice-type moves and knows a Ground-type move. Really recommended.