This is quite easy but it can be luck based too. The idea is, as a plant, to stop the zombies from capturing the very first garden throughout the session. Stop them there and this trophy is yours.
If you're going for this one in particular then always try and swap teams to the plants towards the beginning of a round... if playing as a zombie and you still need this one, if it looks as though you're about to fail then swap teams at the last second! It's a little bit of a cheap trick but hey ho, it can save you from an otherwise frustrating trophy.