Defcorn 5

Strategy
Take the following plants:


 * Spikeweed
 * Garlic
 * Sunflower
 * Kernel-pult
 * Cob Cannon
 * Whatever else you want

Play the first day level, because it contains only normal zombies, it's also possible to use Day: Survival as it gives you plenty of time. Next, use Garlic in every other row (rows 1, 3, and 5), to force the zombies into two rows. Then, place three Spikeweeds at the very end of rows 2 and 4, killing all the zombies. This will allow you three rows of unthreatened Sunflowers. Whenever the Cob Cannon charges, take out two Sunflowers and plant one. See Survival Mode for more information.

You can easily get this achievement in the Mini-game Last Stand as you start of with 5000 sun which is enough to plant 6 Cob Cannons and Tall-nuts in front of them. You can also beat the mini-game with the 6 Cob Cannons.

Note: If there are Gargantuars coming, you could also get the achievement, Popcorn Party in the process.

Trivia

 * This achievement is a reference Defcon, or Defense Readiness Condition, a scale of alert for the U.S. armed forces. Ironically, DEFCON 5 is actually the lowest state of emergency, with 5 Cob Cannons in a normal adventure level being somewhat of overkill.