Let's start with a reality check: A much stronger player (MSP) is going to beat you if he plays his level. That's why he's an MSP. But everybody has "off" days, and if you want to maximize your chances of knocking off a MSP (other than simply improving your own game, in which case the opponent no longer is an MSP), then there are three things to focus on:
- Beat him tactically.
- Maximizing the chances of the MSP having an "off" day.
- Maximizing your chances of beating the MSP if he has an "off" day.
How do you beat a MSP tactically? Simple - study his game so you know his strengths and weaknesses, relative to his level. Then develop tactics so that he has to beat you with his "B" game against your "A" game. Does he have a big forehand loop against backspin? Then perhaps serve short, heavy, low backspin to his forehand, and if he pushes it back, either attack or quick push to his backhand, making him attack with his "weaker" backhand. If he attacks the short serve, then you are making him beat you without his big forehand loop against backspin, his biggest strength. But don't forget to bring your own game into account - you want your "A" game against his "B" game.
How do you maximize the chance of the MSP having an "off" day? First, see the first item above - if you beat him tactically, then he's more likely to have an "off" day. It's hard to be at your best if your opponent won't let you use your best shots! But the other major way is to use lots of variation so the MSP never gets into a rhythm. Mix up your serves, your receives, your pushes, your blocks, your loops, and everything else.
How do you maximize your chance of beating an MSP if he has an "off" day? First, focus on the two items above - beating him tactically and varying your shots so the MSP never gets into a rhythm. But then you need to focus on one simple thing: You must play well yourself. That means staying relaxed and just letting yourself go, but without overplaying that just leads to mistakes. Don't think of the MSP as a "Much Stronger Player." He's just a big fish you've hooked, and so all you have to do is focus on reeling him in. I hesitate to say this, but the biggest cause of an MSP coming back and winning is when the MWP ("Much Weaker Player") realizes he's about to beat an MSP, and mentally falls apart. MSPs are generally very strong mentally - that's why they are MSPs - so you have to be just as strong mentally as they are.
Why did I hesitate on saying this? Because the very act of thinking about beating an MSP in a match is the biggest cause of choking it away, and my mentioning it may make you think about it as well. But you need to prepare yourself in advance of the match against the MSP so that when you are in that position to win, you are mentally ready to play it all the way to the end without thinking about actually winning or losing. So just think of him as that fish you are reeling in, and keep reeling!