Alright, believe it or not, this one is actually pretty easy, even if it is worth 100 GamerScore.
First thing you need to do is to make sure you have the difficulty set to Veteran.
Pick the Atlas Falls mission.
Under no circumstances, do you want to use your flash grenades or regular grenades until you get to the room that you have to breach. When you get to that room, and the little video thing plays where you set up the breaching device, and then get pushed back. Run off to the hallway to the left of the door that you have to breach.
After they shoot through the breached door for a few seconds, throw a flash grenade, wait a few seconds, then throw a regular grenade. Rinse and repeat for all 4 sets of your grenades, but be sure to spread them around.
When I did it, I had to retry it only 2 times, and time that I did get it, I only had 1 guy left to kill in the back.
REMEMBER!!! YOU CAN TAKE NO DAMAGE AFTER YOU GET THROWN, or you will not get the achievement.