Monster Mash

An easy way to get this achievement is to hold up 5 or more zombies against any defensive plant like Wall-nuts,(except Garlic) and planting a Squash when all the zombies are stacked up. Just make sure that the defensive plant isn't eaten first.

Note: Letting five zombies get crushed by a Squash in I, Zombie won't unlock this achievement.

Trivia

 * The achievement is named after a Halloween song "Monster Bash".