Plant two columns of Sunflowers and Potato Mines 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 Tall-nuts closer to the end to complete your defense.
If you want to, replace two Sunflowers with Umbrella Leaves (as shown in the screenshot above) to defend your plants in the back columns from basketballs and Bungee Zombies.
Second flag. Because of his speed, Ladder Zombie would have had some bites on my Gloom-shroom if there is nothing on his way. So I used Flower Pots to prevent my Gloom-shrooms from being eaten.