I found that the best way to do this is to set up a local game with four teams. Set 3 of the teams to a second controller and one to the account you want the achievement on. Set the game mode to 'Full Wormage'.
Once the game starts, teleport all the second controller worms to one place that your account's team can easily attack with dynamite or sheep (sheep usually works great and you can set it off from a distance).
You should get 12 kills per game so it takes about 8 and a half games to get the achievement. I haven't actually gotten the achievement but this is how I got the Shepard achievement and it was definitely really easy. Shouldn't take you longer than 1.5 hours to finish.