The best way to beat a person that slices and chops alot (I bet he hits dropshots too) is to:
1. Anticipate the ball. Pay close attention to his racket face. Usually before slicing, he will square his hitting arm to slice through tha ball.
2. Be quick on your feet. Be prepare the run towards the ball and hit a return.
3. When your opponent slices the ball, it will either be deep or short, and very low to the ground. Once you get to the ball, slice it back deep or angled towards his BH side.
4. Attack the net and cover the angled return.
Again,
Anticipate the slice and chop. Be prepare to run down the ball upon detection. Return the ball with another slice or chop to his BH side and converge onto the net to cover the return.