Mix up your shots. After 2 shots with top spin, maybe slice the ball or hit it flat, hit with top spin, slice, slice, topspin, you get the idea.
Another good way to beat a pusher is to go to the net. If your net game isn't good then you must improve it. Since a pusher usually doesn't hit hard, you have a chance of putting away the overhead or volley if your approach shot is good enough.