This is a bronze achievement worth ten gamerscore

To unlock this achievement you must Score "excellent" in the shooting range in mission two "Atlas." You have to score 2000 points in the shooting range to unlock this trophy or achievement. You must hit as many red targets as possible while avoiding the blue targets. Headshots give 100 points each! Pick a fully automatic weapon that you are comfortable with and start from the spot in the middle of the range (from here you can easily see all targets). Only fire in small bursts to save ammo and reload between rounds.
There are 3 rounds and short breaks in between.
Activate the Overdrive Ability when the voice in the background tells you to do so. You will need to use Overdrive once per round to hit the fast moving targets.You can keep repeating the shooting range as often as you want. If you have too much trouble with this on your first playthrough, then upgrade your Exo for increased accuaracy + reload speed and return later via mission select.