Generalizing all lefties into one category is difficult, but I’ll do my best to explain some general strategies that would apply to most lefties. I’m assuming that you (the reader) are right-handed.
When the lefty is serving a forehand serve from the backhand side, adjust your position slightly more to the right. The lefty will often use a sidespin serve to curve the ball away from your wide forehand. By standing more to the right, you will be able to better cover the wide forehand.
An article related to improving players table tennis skills.