The icon features a Zombie, being crossed off by a "no" symbol.
Simply playing as Plant Hero should net you this eventually. Note that even if you lose the match, so long as you destroyed a zombie, it will count.
Insta-kill tricks such as Whack-a-Zombie, Shamrocket, or Squash will work wonders here, as they essentially offer a guaranteed destruction. Given Wall-Knight can use all three tricks, he's especially useful to obtain this achievement.
- This is similar to the achievement, Master Mulcher.