Rockstar Table Tennis

Not hard, just time consuming. It only counts if the ball is moving so idle time does not.
Either find a boosting partner and rally or use a 2nd xbox with a 2nd copy of Table Tennis.
When you are rallying, use the slice shot (Y) as it is much slower so it's easier not to mess up.

You can make this one in one night, just using you TURBO CONTROLLER with a friend (it's best if both of you have the turbo). Set the Y button on turbo.

In 10 hours you will get the cheevo !!! You can be sleeping while doing this !

You can play 2 hours, finish the match, and than do that 4 more times, so if the Xbox crashes you wont lost the hours you've played.

This is a bit of a Late Solution since I only unlocked it last night... finally. But I thought I'd let you know, that there is actually a way you can track how long you've actually Played Online Multiplayer on XBox Live.

From the Main Menu, click Extras, then click Stats. What you do in the Sub-Menu that comes up is press or all the way down to the bottom of the Page. There you will see how long you have played the Multiplayer for in this game. It is registered first by Hours, Minutes and Seconds.

Also, another thing to keep in mind is this Achievement Unlocks as soon as the requirement has met.

The reason is because I got proper annoyed as I crashed and fell Asleep when I originally had 23 Minutes (9 Hours - 37 Minutes) left to get the Achievement. When I woke up after falling Asleep during the Match I was pressing the button on 2 Controllers, I decided to check my Stats after that match. I had 9 Hours - 59 Minutes - 49 Seconds --- I pretty much needed 11 Seconds to get the Achievement. So I Hosted a Match which would be First to 21, and the Achievement Unlocked during the match, whilst I was taking a Dump in the Toilet. I admit this because when I returned, I opened the and checked my Gamerscore, which was increased by 60G.

Hope that helps and good luck if you are going for this Achievement.