Plants vs. Zombies: Battle for Neighborville

This achievement, as well as its Build-A-Buddy counterpart, is surprisingly easy in the story mode of this game. Either go to "Sundrop Hills in Town Center" by talking to Vivian, or "Mount Steep" by talking to Sir Victor-Ian. You may need to play through the main story for a bit, but once you have freedom to explore you want to find "Mark".

In "Sundrop Hills In Town Center", Mark can be found in Pressure Pier.
In "Mount Steep", Mark can be found in the Rocky Flats.

Once you have found him, you'll want to interact with the glowing Gardenator in front of him. When you hover over it, it should say " Play Ops". From here, just play a few rounds of Ops, making sure to fill the plant pots every time they are killed.
The default Peashooters have no cooldown, so you can spam these around the map to maximise time.