Pretty easy achievement to get if you have a 2nd controller. Simply load up Terrorist Hunt and select Border Town as the map, Low enemy density and then you can start.
On one controller, just go around killing as many enemies as you can, you can play it carefully or not, just make sure that if you die on that controller, then make sure that Player 2 is still alive, and then continue the mission using that controller.
The last 7 enemies will typically rush you, so make sure that you get yourself into a position which you can mow down the enemies easily (Personally I was on the staircase in the building at the middle of the map).
You can also do this online as well, the game has a small community, but you can find someone else to play with sometimes. If not, you can always get a friend to help.