Okay. A few posts ago, I came up with the 1 Cob. Basically played like Cobless, so not too much of a stretch. I've pretty much finalized my idea. I'm to 20-28 flags (somewhere in there, =P) so far, but lots of failed setups can make it that far. I would've used a non-Firepea build, but I wanted to master the Firepea, and now I realize that it makes playing SO much faster. Here it is. (Oh, and even though I designed this, it's based on Draco89123's build Halfway, so I'd like to give Draco credit for this setup.)
The lineup in the photo is what I normally bring. If I'm having a really bad round in terms of sun, I can replace those 2 Ice-shrooms in the back with Sunflowers. Another point I use for placing Ice-shrooms is right in front of that Split Pea, and that can also be used for a Sunflower. (The Split Pea can be replaced with a Sunflower, even, if there aren't any Diggers, Gargantuars, or Gigas.) Also, if you just had a really bad round and need to replace a lot of Pumpkins, if there aren't any Diggers or Gigas, you can replace that Imitater Ice-shroom with an Imitater Pumpkin. (Side note: if I'm bored, I may decide to put Spikeweed in the very back, where the Ice-shrooms are in the image, just to kill Diggers more quickly.)
The Split Pea is good for taking out Diggers, obviously, but it's also great for eliminating Imps, cos the Gatling Peas are pretty much useless when they're being blocked by Imps. The Split Pea is replaceable.
The Glooms in the back are for taking out Diggers, and the Glooms in the front are for general other Zombies. I think Gargantuars are killed no matter what with my current build, but I've only had about 20 Flags to see. I try to use an Ice-shroom on Bungees.
One explosive is enough to take out Gigas (though sometimes you'll want to use a Puff-shroom or two to keep it from smashing your front Glooms.) The Ice-shrooms should keep Gigas occupied for long enough for me to Cherry and Cob Gigas, and Squash is for general backup. If I'm right, this should be Zomboni-proof, save the occasional steamrolled Split Pea or Outer Row Fume-shroom. At the moment, the biggest thing I'm worried about are Jaccidents (the term was originally coined by the writer of some FAQ, I think FallEnd, and I'm not sure if I came up with it independently, or was subconsciously basing it on that.) in pretty much any lane. Before 10 Flags were up, I had a Jack instantly explode in the very top row, so don't think you're safe from Jaccidents in the very early game. Luckily, it wasn't in either of the inner rows!
The Fumes in the inner rows are there to stop Jaccidents from occurring. They won't give you a 100% guarantee, but they do provide an excellent safeguard. Without them, I've experienced 2 Jaccidents in one flag, and another 4 flags later, so I'm really crazy about including those Fumes. With them, I've predicted one Jaccident every 25-100 flags, depending on how lucky you are. The extreme risk has been experienced by other users, but some not so much, so I'm not sure what's going on. Maybe there's a larger risk in the GoTY edition (which I have)?
Try to use constant Puff-shroom spam in the top lane if there are Gigas, so they pause to smash it. If you don't have any Jacks to worry about, bring Flower Pot, too, and spam that. (Alternatively, you can bring Scaredy-shroom or Sun-shroom for the same effect with the same cost and recharge, but I tend to prefer Flower Pot.)
Wish me luck!