You've got the perfect Pokémon already, Servine. It resists Clay's Ground type attacks, and can deal 2HKOs or usually OHKOs if it's over leveled, which it should be. In-game is very easy, and there is less need for bulky Pokémon, because your Pokémon will usually be of higher level. I defeated Clay with a Ducklett I caught on Driftveil Bridge, without training, so it should really be quite simple to defeat him with Servine.