let it be known that this achievement can also be won in the Dead Ops Arcade mode. i think that would be the easiest place to do it as you have unlimited ammo, some VERY powerful weapon upgrades, and the zombies are usually very bunched together. to get them in a line, just run in circles and avoid getting hit and they will gather up in a horde in no time. this tactic also helps for the "rhino" achievement, to dash through 20 zombies at once.
side note: to access dead ops arcade, at the main menu hit the LT and RT back and forth until you break free and stand up from the chair. make your way to the computer in the back of that room and hold X to get on the computer. then type in DOA and then hit enter. that will unlock dead ops arcade, AND a couple more achievements along the way
if you have success with this guide, PLEASE upvote it so that it can be the top solution and to inspire me to write more. thanks!