For me, when I am playing someone who is mostly defensive, I try not to change my game/strategy too much (for example, by trying to hit harder or aiming for more winners) because I find that I will easily beat myself through unforced errors.
Instead, I try to use better court positioning by setting up close to or even inside the baseline for my groundstrokes. When you hit your groundstrokes from a closer position (versus the opponent), you put pressure on the the opponent because your shots are coming back super quick. This positioning also allows you to hit far better angles while still maintaining a high percentage. Also, it is easier for you to transition to the net or toward a short ball for a winner if you start near the baseline.