Go to "personal training", "challenges", "dodgeball", and start the drill. You can skip the warm-up drills if you want to without it affecting the achievement.
For this one, like the other "sudden death" achievements, it DOES NOT matter how well you do while the timer is counting down. Conserve your energy while doing the timed part of the drill.
When the trainer says "get ready for sudden death", prepare yourself. Keep moving side-to-side on the line, and watch out for the red "bulls-eyes" on the wall behind your character. That will give you a head-up of where the ball is going to hit. There is no real trick to this one. Keep moving, duck, jump, and most importantly, don't get hit by a dodgeball!
1) keep moving as much as possible. This helps you "stay on your toes" and you will dodge some balls inadvertently just by doing so.
2) move AS SOON AS you see the red bulls-eye on the wall behind your character
3) save some of your energy during the timed portion of the drill so you can keep going when it counts....in sudden death
You just need to keep moving and not get hit by a dodgeball 20 seconds after entering sudden death. The achievement will pop after a ball hits you and the drill ends (as long as you stayed alive for more than 20 seconds in sudden death).