A great advantage to throw an opponent off is to cut off her ceiling balls.
The key is to not show that you are going to cut it off. Wait and then move after the opponent has committed the shot. There is no need to rush the cut off shot. Let the ball short hop and then swing when the ball is a foot or two off the ground.
Swing with a sidearm and deliver a dink shot. Dink shot into the front wall kills the ball and is almost impossible to return. Swing using your shoulders not your wrist.
If the ball bounces higher than 2 feet, put a topspin on the ball, hittnig the ball downwards to the corners of the front wall.
Topspin shots to the front wall die once they hit the corner and are tough to get. Otherwise the ball will go to the back wall and be an easy hit for your oponent.
Never hit the ball downwards when you are behind the dotted line, you'll skip the ball. The closer to the wall you are the more you use your shoulders in your swing and the straighter you maintain your wrist.
Practice this shot until you feel comfortable, and then use it, mixing it up with other shots, so your oponent can't predict your shots.