I accidentaly found a pretty good trick to make this achievement easier, and since I haven't seen anyone else mention it I thought I should share it.
If you activate Developer Commentary in the settings, then at the top center of the screen there'll be two boxes that trigger commentary by the devs if you click on them. While you're playing the minigame, your mouse disappears but it's actually still active at times (I think it's inbetween rounds). If you manage to move the invisible cursor over one of the 2 boxes and click on it, you'll trigger the dev's commentary; if you do that while the ball is going toward your opponent, he won't move at all and you'll get a point when the dev's commentary is over. Do this 3 times and you'll win.
Granted, this is still a bit tricky. It took me a couple of attempts since I kept triggering the commentary while the ball was moving toward ME (meaning, I lost the point since I couldn't hit the ball), but I eventually got it and I think it's much easier/faster than the method of slowing down the game or winning legit.
If you need more details, I recommend doing this: talk to the guy to start the minigame, and instantly move the mouse over the dev commentary box (if it has a white outline it means the mouse is hovering). Then, when the game starts: if the ball moves toward you, lower the mouse a good amount (try not to move it left/right) and click so your character hits the ball, then raise it to hover the dev commentary box and click it while the ball moves toward the other guy; if the ball instantly moves toward the opponent, just click the dev box. Rinse and repeat 3 times.
Again, the timing's tricky but this is by far the best solucion imo.