This is very easy to get the first time you come up against a Gargantuar.
The first level that one comes up against you, it will be the last enemy to appear on a rooftop level.
The trick is to buy the rooftop cleaners from Crazy Dave for $3,000 before you start the level.
If you do this, the Gargantuar will get to the end of the row and get run over by the cleaner. The added bonus is that this is the end of the level too, so you don't have to worry about rebuilding the plant pots.
If you have any questions, leave me a comment and I'll get back to you.