This achievement is straightforward. I got it on the first level a chomper is available (1-8). Start by planting 1 sunflower at the far left to accumulate sun points. Place a chomper toward the left side of the lawn, with a wall-nut one space to the right of it, starting in the row in which the first zombies appear. As points become available, add 2 more sunflowers on the far left, and place the same chomper/wall-nut combos in each row. As the level progresses and more zombies come at once, put an additional chomper in each row, a little farther right, again with a wall-nut in front. This will slow them down enough so the original group of chompers can finish their meals and won't get eaten. I did not encounter any of the pole vaulting zombies in this level and did not have to use a single lawn mower.