Play level 1-1 Slow Starters as Assaulter.
Clear out the first floor by kicking doors (Y) to stun and using Aim Shot (LT) followed by RT to kill any additional enemies. Rescue all hostages.
Go upstairs and clear out the right side. There should only be one enemy in there. Wait until he is next to the door, kick it open, and aim shot to kill. Rescue the hostage and your Sniper Support should be filled. Go left and use it to clear out the left side room, and then rescue the hostages. If you did not get hit at all during this entire process, you will get the achievement.
Here's this in video form: