Popcorn Party

Strategy
In the Windows and Mac Game of the Year version, just play Survival: Endless and build your Cob Cannons early in the game. When the Gargantuars appear, shoot them using Corn Cobs. You can also play Survival: Night (Hard) to earn this achievement. Two or more Gargantuars will come each wave starting around the fifth wave. Since the Gargantuars come in clusters, this achievement can be easily earned.

In the iPad 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 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 too 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 flower, 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.

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