The set up:
Spike Rocks - I think everyone will agree they are necessary in front of the cobs because they pop the tires of the Zomboni's and slows down the Gargantuars. My strategy has them in the back as well because they kill the Digger Zombies. This is a MUST. I have tried the setups that put Gloom-Shrooms back there instead, but I was always having to replant them. With this set up, you will never have to replant a spike rock in the back, and the Digger Zombies don't give you any trouble.
Gloom-Shrooms - In the first two pictures you see Gloom-Shrooms in the next two rows after the Spike Rocks in the back. I haven't seen anyone else do it this way before. Most other setups I have seen have had more Winter Melons back there. However, this is the perfect set up for me when it comes to killing the Digger Zombies, as well as the Imps. Additionally, when the Bungee Zombies come down, the Gloom-Shrooms kill them before they can grab a plant. At least they do when there's 4 of them in the back like that, and when you use an Ice-Shroom. I had less replanting with this set up than anything else I've tried. And of course, I planted Gloom-Shrooms in the first three rows in the water. I haven't seen a set up that beats having these in the first three rows of the water yet.
Winter-Melons - In the fourth row in front of the double row of Gloom-Shrooms I planted Winter-Melons. I discovered that only one in each row is enough to slow down the Zombies.
Cob Cannon - My set up uses eight (8) Cob Cannons.
Ice-Shrooms - In the water in front of the two Cob Cannons I save room for two Ice Shrooms. I used these on almost every level, keeping those slots full so I could easily drop a Coffee Bean on one quickly at the appropriate times. The only levels that I didn't utilize these as much were the levels with the Zomboni's because they don't slow these down. But I definitely used them any time there were Bungee Zombies, and had one on hand to use when my Cobs were all used but I needed to slow down Zombies for a few seconds to allow the Cobs to recharge. Priceless. This saved many plants. These are a must. If you use these right when the Bungee Zombies are about to come down, it freezes them long enough for the Gloom Shrooms and Cattails to kill them, rendering the Umbrella Plant pretty much useless.
Umbrella Plant - The only levels that I found these to be useful were the levels with the Catapult Zombie. You can see in picture 4 that I have these planted. I waited until the Catapult Zombies killed a plant in the back and then dropped an Umbrella Plant into that slot to protect my plants for the rest of that level.
Sunflowers - You don't see any in any of my pictures. Setting up, I plant many of them at first and get my Suns up to the 9000's. After that, if you play the game smart, you really don't need any Sunflowers. The sun falling from the sky is enough. However, if you see the need to plant one, what I do is occasionally after using an Ice-Shroom I would plant a Sunflower to bring my Sun back up to over 9000, and may also plant one in the spot where the Umbrella Plants are. Once my sun is maxed out, I usually will replant a Gloom-Shroom in that spot.
Cattails - I find it necessary to have two (2) of them. You may be tempted to use only one, and plant a Sunflower in the other spot. However, only one Cattail is not enough to handle the Balloon Zombies, and after several levels of losing Cobs to Balloon Zombies who dropped out of the sky onto my Cobs, I realized I needed two Cattails. Remember, Winter Melons and Gloom-Shrooms do nothing to stop or slow down Balloon Zombies.
Other Plants: I got bored in picture three (3) and tried to plant Gatlin Peas. Big mistake. I almost lost out on that level as the Zombies overwhelmed me. It took me quite a few levels after that to recover from that mistake.
When Jack-in-the-Box Zombies are on a level, I restock with plants that are in the front of the screen because if I don't kill them fast enough, they can wipe out a whole row of Gloom-Shrooms and Spike Rocks in the front, so I make sure to have those on hand to replant quickly when needed. Again, Ice-Shrooms are great against these. They freeze them until you can kill them. It's much cheaper to use an Ice-Shroom than to have to replant three Gloom-Shrooms, so use them!
More Tips - I use the Pumpkin on every level, and often use the duplicator, or replicator, whatever it is called, to have a second Pumpkin. Much cheaper to plant one of these around a plant than to have to replace the plant. You can plant one on top of a damaged one, so do that whenever you see one damaged.
Having the right set up and choosing the right plants for each level is only part of the skill needed to play into the high levels. It takes a lot of practice with the game to know which plants to choose each level, and when to deploy your cobs and other plants to blow up or freeze zombies. It is a skill that will only come when you put time into playing a game. Someone could set a nice piano in front of me and I still couldn't make it sound good. Only someone who has spent hours practicing can do that. It's the same with anything. So keep practicing! You will get better and better.
What's your best strategy? How far have you gotten? I would like to know. Reply below, and I may challenge you with your set up and see if I can beat your high number! If my game doesn't shut down on me again. Grrrrr.