Yeah I played a guy like that and beat him for a rare win. The secret is to move him around because he will be super slow. So use topspin control shots to the corners to set up a flat power winner. Most of the times these guys will be guessing where you are going and cheating that way, since it is the only way they can cover the court. So you want to wrong foot them a lot. This will also keep them from being able to load up on a power shot.