One of the most difficult situations a player may face on the tennis court is what to do when your opponent is capable of hitting a large variety of shots. This includes changing the pace, different spins, different trajectories, and different depths of shot.
Your goal should be to try to hit your regular shots regardless of the type of ball they are hitting to you. This might mean adjusting the speed (slightly) you are hitting or hitting to a bigger target area. The variety tends to make you hit the ball harder or softer, and this can throw off your timing resulting in contact that is not as solid. Watch the ball all the way in to your racquet and hit your normal shot to a spot that is not close to the lines.
Halfway between the middle of the court and the sideline, as well as halfway between the service line and the baseline should give you enough room to make your normal shot the majority of the time. It is sometimes easier to volley the ball when you are struggling with this situation since you do not have to deal with the different bounces that result from a player with a lot of variety.
Try to keep your normal ideas with just a few adjustments.