This achievement is given for, obviously, hitting 75% of your shots through out a 3 round challenge. (After 3 rounds a conclusion screen will pop up. Its the screen which says red/blue team wins.)
Its virtually impossible to win and get over 75% accuracy so you're pretty much doomed to a loss if you want to get this. Here's what I did. Simply shoot the first target of the level once and then sit there. Do this for all 3 rounds and you're set. Achievement unlocked. The other team will indefinately get 'True SSC Challenger' if you use this technique so make sure you have someone who will return the favour.