Lawn Mower Man

Strategy
You can do this in Wall-nut Bowling, Wall-nut Bowling 2 for a bigger chance of obtaining the achievement.

get this on Co-Op mode.

All you have to do is use 4 lawn mowers to mow zombies, before the Flag Zombie comes. A message "A HUGE WAVE OF ZOMBIES IS APPROACHING!" appears and there will be "FINAL WAVE". You have the last lawn mower and you can't tell how many there are. Count the zombies how many you see and don't use a Wall-nut or you can't achieve this. The lawn mower will destroy them and you will get this achievement, along with "Last Mown Standing" and if possible, "Close Shave" as well.

It's also possible to hold off multiple zombies in one lane with Tall-nuts until 10+ zombies appear, and then letting them through to the lawn mower. You can use some Garlic to get more zombies get in the lane. (The best way is put Tall-nut in lane 2nd or 4th, Garlic in 1st (5th) and 3rd (the Garlic in 3rd lane may not divert zombies to the lane has Tall-nut).