Here is the full list of all 10 Kinect Sports Gems: Reaction Rally achievements worth 50 gamerscore.
Reach level 1 (or observe another player doing so) to gain the rank of Racket Juggler.
Reach level 2 (or observe another player doing so) to gain the rank of Hard Court Heavy.
Reach level 3 (or observe another player doing so) to gain the rank of Forhand Fiend.
Reach level 4 (or observe another player doing so) to gain the rank of Backhand Slicer.
Reach level 5 (or observe another player doing so) to gain the rank of Top Lobber.
Reach level 6 (or observe another player doing so) to gain the rank of Multiball Menace.
Reach level 7 (or observe another player doing so) to gain the rank of Return Champ.
Reach level 8 (or observe another player doing so) to gain the rank of Yellow Flash.
Reach level 9 (or observe another player doing so) to gain the rank of Red Flash.
Reach level 10 (or observe another player doing so) to gain the rank of Tennis Titan.