Defcorn 5

Strategy
Take the following plants:


 * Spikeweed
 * Garlic
 * Sunflower
 * Kernel-pult
 * Cob Cannon
 * Imitater Sunflower
 * Twin Sunflower

As 5 Cob Cannons require 3500 Sun, it is suggested you play the first day level, as it contains only normal zombies, and is easy to beat. It's also possible to use Survival: Day. Use Garlic in every other lane (1, 3, and 5), to force the zombies into two rows, placing three Spikeweed behind them to incur damage. This will allow you to build up three rows of unthreatened Sunflowers. The, whenever the Cob Cannon charges, you can take out two Sunflowers and plant another Cob Cannon. See Survival Mode for more information.

You can easily get this achievement in Last Stand as you start off with 5000 Sun, enough to plant 6 Cob Cannons and two Wall-nuts in front of each. It is not necessary to complete the mini-game to earn the achievement.

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

Strategy 2
Take the following plants: Opt for the first day level, as mentioned above. Start out as you normally would, planting sunflowers on the row closest to the lawn mowers. Upgrade your sunflowers to twin-sunflowers throughout the level when you can. Plant peashooters or Cabbage-pults in the row after your sunflowers (plant according to which lane the zombies come into first). Plant Kernel-pults in the third and fourth rows from the back and plant Wall or Tall-nuts in the fifth row. Place pumpkins over the Wall and Tall-nuts if you can afford them. It is best to have both Wall and Tall-nuts in your arsenal so you can purchase one while the other is still recharging. Progress through the level as you normally would, upgrading your Kernel-pults to Cob Cannons whenever possible.
 * Peashooters of any kind or Cabbage-pults
 * Kernel-pults
 * Cob Cannons
 * Imitater Sunflowers
 * Twin Sunflowers (Optional, but provides more sun)
 * Wall-Nuts or Tall-Nuts
 * Pumpkins (Optional, but provides additional protection for your Wall and Tall-nuts)
 * Any Insta-Kills you want
 * [Wall-Nut First Aid] to avoid having to dig up and re-plant Wall and Tall-nuts

When you reach the final wave, try to take out all the zombies in every row except one (one that has a Cob Cannon in it). Leave one (or two) zombies in that lane and immediately remove all peashooters or Cabbage-pults. Now nothing should be attacking the zombie(s) left in the lane, and they should be noshing away on the Wall or Tall-nut [and pumpkin, if you have one]. It is best to have only one zombie left, as this will reduce the Wall or Tall-nut's rate of deterioration. Now steadily collect the sun produced by your sunflowers, occasionally re-planting a pumpkin or planting a Wall or Tall-nut over the previous one. Upgrade your Kernel-pults to Cob Cannons when you have enough sun, use a Corn Cob Missile to take out the remaining zombie(s) when you have all five Cob Cannons, and then you're done!

Trivia

 * This achievement is a reference to 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, while 5 Cob Cannons in a normal Adventure level being somewhat of overkill. Possibly it means the lowest state of emergency for the plants, or more truly, for the house.
 * In iPad version, the achievement icon is upside down.
 * This is one of the two achievements involving Cob Cannons, the other being Popcorn Party.