Ring the red bell in both rooms and survive the attack.

If you're trying this straight after your Veteran playthrough, you'll need to launch any mission apart from S.S.D.D. on Recruit difficulty and save and quit, otherwise you'll be on Veteran.
Once that's done grab the ACR and M4A1 from the shelf in the middle of the room. Then press the Red button on the desk. Once it's pressed run down the corridor and camp in the door way of the room on the left side. Sit here until you've wiped out all the enemies. When the music stops playing you've killed all the enemies for that wave.
Then press the Red button in the second room on the desk and repeat this process. Once you've cleared the second wave the achievement will unlock.



Achievement is worth 30 gamerscore.