The end state of my configuration usually ends like this:
In round 1, I start with two rows of Twin Sunflowers in the back, and a single row of [[Kernel-pult]s. Then I work on getting a Melon-pults in row 4, and the three magnet-shrooms in row 5. I use Potato Mines and Pumpkins/Squash to stop anything that reaches the front lines.
In round 2, my goal is to finish the twin sunflowers, upgrade one row to Winter Melons, three rows of melon-pults (replacing the kernel-pults), and get Umbrella Leaves up in row 5 (and possibly row 2 if Bungee Zombies are present). From here to the end, I begin using the explosive plants as needed.
In round 3, I plant a Gold Magnet. This is optional, but I'm lazy. Also keep only a single umbrella in row two and swap the other one out for sun production when possible. Start covering your middle rows with pumpkins to stop thrown Imps, and finish the winter-pults.
In rounds 4-5, finish the pumpkins. Then save up 1600 or so sun, and replace the back row with melons as the final step.