20 Below Zero

Difficulty: Easy-Medium

Overview
20 Below Zero is an achievement found on iPhone, iPad, PS Vita, Android, and Nook. It can be earned for freezing 20 zombies with an Ice-shroom. It is easy to get in Last Stand and Survival: Endless.

Strategy
One way to do this is in Survival: Endless, build a good strategy that can reach over ten flags. When there is going to be another small wave coming, use the Ice-shroom to freeze the zombies because it can freeze some zombies that aren't on the screen yet. If you do not have Survival unlocked and you don't want to wait, you can probably unlock this on the final wave of the last night level. Another strategy would be putting defensive plants in all rows until the zombies count up to twenty. It may also be possible to achieve this in Whack a Zombie at the final wave. Removing lots of graves will help since a lot of zombies will ambush at your house. It is quite possible to get this achievement in the night levels.

Another Strategy
Another way to beat this achievement is to go to the Mini-game Last Stand put several rows of Tall-nut and Pumpkins out and after 20 zombies walk out use the Ice-shroom, or just simply stall the zombies until there's 20 of them and use the Ice-shroom.

Yet Another Strategy
This strategy can be used in the Day levels 1-1 or 1-2. Load out with the following: Go through the level as you normally would, planting Sunflowers and Peashooters. At the final wave, use the Shovel to clear your entire lawn, plant an Ice-shroom near the back of your lawn, and wait for the zombies to populate the lawn (there should be about 20). Once all the zombies populate the lawn, wake up the Ice-shroom and let it do its work. After that, you can plant a Doom-shroom in the middle of the lawn, and then wake it up to clear the field and complete the level.
 * Sunflower
 * Peashooter
 * Coffee Bean
 * Ice-shroom, the main component
 * Doom-shroom, for some serious field clearing
 * Anything else.



Trivia

 * This is the only Achievement to have it's first word a number.