You mentioned adding backspin. Backspin is opposite of what you want to do. YOu need to strike over the ball. Backspin causes a ball to sail so if you hit it hard it will either go into the wall or over the goal. The only way to hit a ball with backspin and not have it go over the goal is for you to hit it softly. Sounds like that is the problem. A ball hit with topspin will dip down which is how you can hit a ball that goes over the wall and still be hard and not sail over the goal.
