• You must earn all 1000 points IN A SINGLE SESSION of practice mode. It is NOT cumulative over all your gaming sessions(e.g. scoring 100 points a day over 10 days, and turning the game off in between each day will NOT give you the achievement). It is not even cumulative over a single session of the software/game. If you exit practice mode, play a different mode for a bit, and come back to practice mode, the counter is reset.
  
• There is a delay with Steam awarding these achievements. Sometimes they will not award until after you exit the game. This might make you nervous if you pass the threshold for some of the other achievements along the way and don't see the pop-up. Rest assured, the achievements will get awarded after you exit the game. If you earn multiple (50, 100, 200, 500, 1000) in one go, you will get awarded all of them as you exit the game, at the latest.
  
• This achievement can be earned on easy mode. Don't try this on medium or hard. There's no reason to. It will just make it take longer. It already takes too long.
  
• It took me about an hour to earn 200 points. I consider that average, I'm not the best nor the worst at table tennis. So plan for this to take roughly 5 hours of total play time. I could not do this all at once. I played it for about 30 minutes to an hour at a time, set the controller down, left the game still running in practice mode and came back to it later. I stretched that over 2 days, leaving the game running overnight even.

As for beating the AI, it's a little weak on returning any angled shots, but other than that there's no real secret. It's just persistence. Between point 1 and point 500 you will probably know the AIs weaknesses intuitively by then.

Proof:
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=2955722242