Popcorn Party

Strategy
In the iPad/iPod Touch/iPhone version, you can play the first level where Gargantuars appear in Quick Play. Bring the Cob Cannon and Kernel-pult, as well as Sunflowers, Twin Sunflowers, Cabbage-pults, and Flower Pots. Dedicate the remaining seed slots to a combination of defensive plants, such as Wall-nuts, and instant kills (though preferably not the Potato Mine).

When playing the level, start with your Sunflowers normally, adding a column of Cabbage-pults in the furthest Flower Pots. Upgrade your Sunflowers as you can to Twin Sunflowers, then build as many Kernel-pults and Cob Cannons as possible. Continue doing this throughout the rest of the level. Try to not to rely to heavily on the Cob Cannons, however, as you must save corn cobs for the Gargantuars when they appear. But don't shoot a Corn Cob at him when he's in the third column as he'll toss his Imp to eat every single Sunflower, but use 2 Cob Cannons to fire at him before he tosses his Imp. You need to be fast to fire the second missile immediately after the first one or the Imp will be thrown, attacking your plant at the back.

When a Gargantuar does appear, weaken it with an instant kill, and then quickly fire one of your Cob Cannons at it. Do the same thing for the second Gargantuar, and you will obtain the achievement with ease.

In the PC version, Survival Endless is a good strategy. Do Sunflowers like you normaly do, then plant the Kernel-pults, then any other plants of your choice. When done, if you have enough for Cob Cannons, go for it!

Any Survival hard mode can work the same way.

Note: It is also possible that you can obtain the achievement "Defcorn 5" by using this method.

The iPad minigame Buttered Popcorn can't make you achieve this because there's only 1 Gargantuar.

Trivia

 * This achievement can be obtainten by killing two Giga-Gargantuars
 * Playing Buttered Popcorn will not get you this achievement.
 * The Reason is because there is only one Gargantuar.