Ugh, curse this game.
I beat her 6-1. Your turn, Deguello.
All those chumps <1490, I pretty much used the same strategy to exploit their running inability (like the player has to put up with), and get clean 6-0's.
For this cheating AI trash, I ended up finding a different exploit. Used immediate sharp angle top-spin returns that puts the pressure on her, reduces her available reaction time, prevents a long range match, preventing her from taking advantage of her unfair moving ability and surprise shotgun returns, as I make her chase the ball past a corner.
Your return has to have a good enough angle and speed that she won't have time to move to whatever her bullshit calculated ideal spot would be to make a controlled return, forcing her to lob, and it's up to you win the subsequent smash/lob rally. You're hitting back so soon during the rally, you're Mii is automatically staying close to the table so there's less worry about moving slowly and getting stuck at corners. Having to go to these lengths is ridiculous.
And the last key part for all this: don't make a mistake.