Last Mown Standing

Note: This can easily be achieved, but remember, when you see the last Zombie, don't defeat him or you won't get it.

Strategy
Time to start Adventure Mode. Just start a level like you normally do and when it says "Final Wave", remove all the plants on one row and the Lawn Mower will kill those zombies and you'll get this achievement.

Note: Defeating other Zombies with a Lawn Mower won't count towards this Achievement.

Trivia

 * This achievement's name comes from the phrase "last one standing", usually used in children's outdoor games.