This can be done in custom games
On the main menu hit X < then click on your gamertag < click on find player < type in "That505Guy" < open file share < download map file Assist Map < download gametype Assist GT
Load up three controllers < Make one a teamate < start game.
The enemy will spawn on ledge. Place your guest teamate on the turret. Grab the needler on the achievement account pick up a second one stay in place for ammo. Change your settings to classic to disable aim assist, turn off vibrate.
Both accounts aim for the spawn point on start shooting. Alternate needlers as to always have constant fire. Do this for about 1 hour and you should have your achievement.