Okay, guys, a few days ago in the chat, Drek'TharSuperSword suggested I try this build.  As per this, I am trying Halibabica's build Antipode.  Disclaimer, this is not my build.  I'm to forty-some flags so far.

Halibabica's Antipode

A brilliant little build, it's Halibabica's, not mine.

As for an explanation of how to play it, I'll quote the designer on the Steam forum thread where a heckuva lot of the pioneers of Survival Endless worked.

Gatling Pea and Torchwood - once thought to be useless in Endless mode, this pair isn’t entirely unworkable in a build. It’s important to understand how exactly they work. It’s obvious that the fire peas do double damage and that Gatlings spit them out very rapidly, but they only hit the first zombie in the row, right? WRONG. When a fire pea collides with a normal zombie (normal being anything that isn’t made of metal), it has a fiery splash effect that damages all other zombies nearby the target. The fire counteracts the effects of snare, but only on the initial target. Zombies hit by the fire splash take one point of damage and remain slowed from snare. This means that Gatlings shooting fire peas can hit multiple zombies at once, making them a plausible addition to an Endless build. But, there are some zombies that can counter the Gatling + Torchwood combo which must be watched out for. When fire peas collide with screen doors, ladders, catapults, and Zombonis, they lose their splash effect and only do damage to the metal target that took the hit. This is the primary reason Gatlings and Torchwoods fail in Endless, and the reason why you can’t rely on them alone. I’ll repeat that: YOU CAN’T RELY ON THEM ALONE. Gatlings and Torchwoods output tons and tons of damage; enough to drop a Giga-Gargantuar in about fifteen seconds. But they can’t stop everything, and if you’re going to use them, you need to have a plan for how to take out the metal zombies and everything behind the enemy front lines. Otherwise, they get backed up and you have a huge wave of zombies knocking on your door before you know it.

Halibabica’s Antipode Build

This unique Endless build implements both fire and ice to take down the zombie hordes. The pool and inner rows are protected by the typical Gloom setup, and the outer rows are handled by a combination of ice snare and fire damage. The cob cannons are best fired at fifteen second intervals, and always on the zombies furthest in the back. It’s tempting to shoot zombies near the Spikerocks, but anything in that area will be dead within moments anyway (and Gargantuars of both kinds can still be hit comfortably with a blast further back). Protecting the Spikerocks is very important, so you’ll want to save your massive damage consumables for Gargantuars that get too close. The Gatlings can handle single Zombonis, but multiple ones in the same lane will need to be dealt with. Plant Spikeweeds directly underneath them to pop all their tires at once. If a Spikerock and Torchwood get destroyed, don’t panic. Zombies in that lane will no longer be thawed out by the fire, and the inner row Gloom will help you keep things under control until you can replace the lost plants. When using this build, there are six seeds you should always bring: Lily Pad, Fume-Shroom, Coffee Bean, Pumpkin, Imitator Pumpkin, and Squash. You’ll want Lily Pad in case something in the pool gets destroyed. If nothing else, they’re good for Dolphin Riders to jump over. Fume and Coffee Bean are necessary because the inner row Fumes will get chewed up by Football and Balloon Zombies on a regular basis. Putting Pumpkins on them will get them smashed by Zombonis and Gargantuars, so don’t bother. Both types of Pumpkins are needed to maintain the defense of the yard’s other plants. Imitator should be used as much as possible; save normal Pumpkins for plants in immediate peril. Finally, Squash is a low-cost, fast-recharging consumable that will save you a lot of headaches. The other slots can be filled with consumables or plants that need replaced, but depending on the zombie horde, there are other plants you should bring.

If there are Jacks, bring Glooms. No matter how many precautions you take, Jack can still blow up the poolside, and that’s something you MUST be ready for. Those Glooms protect four lanes, and losing them for too long could cost you your brains. So, always bring Glooms when Jacks are involved.

If there are Zombonis or Gigas, bring Spikeweed and Spikerock. You’ll need the Spikeweeds to pop tires or replace lost Spikerocks. Squash is also very handy for removing groups of Zombonis. If you’re not sure how many Zombonis are in a lane, look at the dents on it. If you can see another Zomboni behind it (that isn’t damaged), then pop / squash ‘em. Gigas are so ridiculously tough that they’re the only other zombie that even has a chance of reaching your Spikerocks to smash them (normal Gargantuars typically won’t get that far), so you should bring extra firepower for them if you can. Cherry Bomb and Jalapeno both work great.

If there are Bungee Zombies, bring Ice-Shroom. You will need to use Ice-Shroom with good timing at the beginning of every flag to make sure the Bungees don’t steal any plants. The yard is set up in such a way that no frozen Bungee could ever survive, save for ones attacking the inner row Glooms (which still die most of the time anyway).

If none of the above zombies are present, you won’t need to bring the plants associated with them. Fill the extra slots with consumables or plants that need replaced. If you get a round with all of the above present, you’ll have the seeds shown in the picture to work with.

Okay, me again.  I pretty much stay to Halibabica's suggestions on what to use.  As for empty slots, I take Cherry Bombs, then Doom-shrooms.  Because I like a gigantic explosion with the word DOOOOOM!!!  I seem to be using about a 2-2-CS rotation most of the time, with extra plants to hold off the Gigas that don't get blown to shreds, but I'm mainly just guessing.  I suggest guessing a lot.  If you try to get your timing perfect, you'll just end up waving bye-bye to those Spikerocks every 10 seconds, apologizing for the fact that theirs is the millionth life to end to save your own tail.