The hardest lesson I've been learning lately: Don't play on auto-pilot!!
Part of your mentality, should be making sure you're using your brain with your hands, and not just your hands. Stay alert and don't just mash through the same pressure strings/combos over and over. By the 2nd match, a good player will adjust while you're still playing on auto-pilot and suddenly you'll find yourself in the last round of match 3 wondering what happened to that win you thought you were going to get.
Also on that note, be very observant of what patterns the other player has. If you make someone block A attack, what do they do? Do they jump, counter, or continue blocking? If they countered, with what did they counter? Set up that situation again with your knowledge of what they've been doing all match, and have B attack ready to ruin whatever you've noticed they do.