Monster Mash

Difficulty: Easy-Medium (Very Easy if Survival Levels)

Monster Mash is an achievement unlockable on iPhone, iPad, Android, Nook, and PS Vita versions of Plants vs. Zombies. Monster Mash is earned by killing five zombies with a single Squash. The player should try to let some zombies stack in front of a defensive plant, dig it up and plant a Squash there. This is fairly easy to accomplish in Last Stand and Bobsled Bonanza by placing a Wall-nut and wait for two Zombie Bobsled Teams to come in the same lane. Then place a Squash behind the Wall-nut and 8 zombies will be killed by a single Squash.

Another easy level you can do this is Column Like You See 'Em, as there is a very high concentration of zombies.

Strategies
Start with any of the survival levels. Recommended is Survival: Pool (Hard). Survive normally till the last two flags.

Plants needed for the last two flags in Survival: Pool (Hard): Have at least five Twin Sunflowers on the screen and collected around 1000 sun and is neatly taking care of other rows. Make sure any one row (recommended the first row) is all cleaned. No plants. Plant one Potato Mine in the first column and protect it with Pumpkin. Next, plant a Wall-nut or Imitater Tall-nut in the second column and wait for zombies to come. Blast those zombies with Cherry Bomb. Next plant a Doom-shroom (without a Coffee Bean) and protect it with a Pumpkin (as far as I know, the Pumpkin would have already recharged by this time) on the 3rd column. Now, when the group of zombies have been gathered near the Doom-shroom give it a Coffee Bean. Plant a Tall-nut on the fifth column. Many zombies would gather to eat it. Protect with a Pumpkin. Now is your time use a Squash when a lot of zombies have been gathered. Plant it on the fourth column. By this time, you should have easily earned the achievement. No need of completing the level.
 * Tall-nut
 * Squash
 * Imitater Tall-nut (or Wall-nut if you don't have the Imitater)
 * Cherry Bomb
 * Potato Mine
 * Pumpkin
 * Doom-shroom
 * Coffee Bean

Strategy 2: Tall-nut and Squash
Needed:
 * Tall-nut
 * Squash
 * Any offensive plant
 * Lily Pad
 * Mode: Last stand

Key: Here is the map:
 * S= Squash
 * T= Tall-nut
 * C= Peashooter
 * = Lily Pad
 * /= Free spot

S/ / / / / / T T

C/ / / / / / /T T

(C)/ / / / / / / (T) (T)

(C)/ / / / / / / (T) (T)

C/ / / / / / / T T

C/ / / / / / / T T

Let zombies pile up in lane 1. When they reach the end, Squash a whole lot of zombies, simple as that. When Squash is done, replace with C to win Last Stand.

How to do it - Last Stand Stratergy
Do the order for every lane.

Map:
 * W = Wall-nut
 * S =Squash

SWWWWWWWW

In Last Stand Endless for iOS, it's possible if you play 15 or more flags normally, then you can sell some undamaged plants for sun, then you place a number of Tall-nuts to stack the zombies up and a Squash when there is a huge pile of them. You can then get the Monster Mash achievement (though likely at the expense of your streak)

Level 3-10
Place a Tall-nut, wait for the mass to appear. Plant the Squash at the final wave, then you get it.

Survival: Endless Strategy
After reaching around 50 flags, you can simply plant a Squash randomly in the mass of zombies, and you will get this achievement.

Trivia

 * The title of this achievement is a reference to the show "Monster Mash" and the song of the same name.