Use the Hero Plant to kill the first zombies and complete the objective at the same time.
Plant two columns of Sunflowers just like in the beginning of each level.
Plant one Kernel-pult in each lane to slow down some zombies, mainly the Ladder Zombie. Then, plant Cabbage-pults and Wall-nuts closer to the end to complete your defense.
If you want to, replace two Sunflowers with Umbrella Leafs to defend your plants in the back columns from basketballs and Bungee Zombies.