Well, seeing as the first gym is a rock-type gym, I would suggest getting your Turtwig up a few more levels. A great place to train with Turtwig is the cave before Orburegh City. It's full of rock types, so Turtwig can easily power through them. But, you would have to keep a few levels up on Shinx and Starly, so train Starly at the Orburegh Mine. It's full of Machops, so Starly can also power through them. But with Shinx, there isn't really a specific place to train. I would suggest getting Turtwig up to level 16 to face Rorark's Cranidos
It has very high attack, so you should definitely kill it off quickly.