In Stage 3, you'll come across some segments where you'll awaken in the seat of a combine and are able to drive it. Use this combine to drive over the wheat while trying to save a few zombies at the end as you get near the end. Mow along the outside of the entire field and take time to back up and resume on the edges, creeping closer to the middle. Once you've mowed the last, you'll see no other wheat and, of course, you'll see the trophy pop up a few seconds afterwards and you can clear the last of the zombies in the field.
There is some hesitancy on this trophy, too, as it is missable but reclaimable if you finish the chapter without it. The difficulty comes from getting to that point in the chapter, just for that trophy.