The other guides on here work as well, but I just thought I would give my way that I completed it with. So, as the other guides say, load up Stonehenge on local play with 2 controllers. You can do this with the A.I. ghost companion, but they aren't always reliable and saving them for revival is what kept me going on round 20. You also want to have custom settings with double health and fast regen. On Stonehenge, you want to go to the two story building with both controllers (or all people) and have on person sit in left hand corner of the second floor as if you were looking at the floor from the very top of the stairs. This person (or second controller) should stay here the entire game and you should camp on the stairs. The enemies will then come into the building and turn to look at the guy on the second floor and you are then able to shoot them in the side without getting shot at by most of the people. Once the enemies get to the bottom of the stairs they will start to charge at you, so try to not let that happen.
You can essentially use any weapon you want, but I actually went through the entire game using the pistol because it regenerates ammo on it's own. I'm not sure if I would recommend this thought because it got really hard in the later rounds. I would probably recommend using the FP6 shotgun and try your best to get slight of hand so that you aren't held up by reloading.
So just keep using this same strategy until you get to round 20 and the achievement should pop.
NOTE: Whatever you do, do not let the riot shield guys get to the bottom of the stairs to charge at you, because around round 18 they will screw you over so much and could cost you the entire game. Try your best to kill them while they are turned looking at the person in the corner.
Hope this helps!