One of the most reliable techniques to tire out your fat opponent and get a bunch of easy points is to learn to serve wide. You don’t even have to hit the ball that hard when serving wide. In fact, it’s easier to serve wide the slower you hit. If you’ve got a crosswind on the court, serving slow and wide is even more effective because the wind effects a slower moving ball more than a fast moving ball.
The whole point is to make your opponent run 3-5 steps outside the singles court and then you hit to the open court which they will never be able to cover.
Get a bucket of balls and practive serving out wide on both the deuce and ad courts and you’ll get tons of easy points.