DE Square Cobless

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Variable tempo:    • DE Square Cobless (Variable Tempo)  
Fast tempo:    • DE Square Cobless (Fast Tempo)  

Strategy by 烈焰鸽斯拉, optimized by 炽热 and 雪枫零奈
aka DE Frosty Winter Cobless (凛冬无炮)
Slow tempo sequence: Card 3: Doom | Ice - Ice | Jala + Squash + Cherry

The key is to ensure all Football Zombies move at similar rates. Therefore, all can be frozen within the Gloom range. Because WM may slow zombies unevenly, two Ices are used.
From my experience, the earliest timing for the second Ice is when the right hand of Football Zombies has completely entered Column 6. The latest timing is when the right hand enters Column 5.

For the second half, you can be more flexible if there are no Gigas. Just use one Ice each wave against the Footballs. If not all can be frozen in the Gloom range, I would prioritize freezing the fast ones and stalling the slow ones. In emergency cases, use Squash/Cherry/Pumpkin.
On wave 19, I used Doom for wrap-up.

Don't worry about Ice preemptively freezing the next wave. Just play normally. Stall the Gigas from the previous waves, and delay the instant timings accordingly.

For slow tempo, Jala can usually be placed right after the new wave spawns. The Doom will usually recharge before the next wave. Ensure the Jala hits the slow zombies (Coneheads, Bucketheads, etc.), or they can eat up the Doom. It's good practice to stall Gargs on row 2 together, so WM can slow all of them, and the Doom will be safe.
Doom timing on 3-7 should be slightly delayed so you don't miss Football Zombies, as on 6:35, but don't delay it too much that Jacks/Dancers can kill it.
Squash should be timed earlier to avoid Pole Zombies, but don't be too impatient as I did at 4:53.
Cherry should be timed late to hit as many Gigas as possible, especially on wave 9.

Major threats:
Giga: All rows have level 2 DPS. With Ice, it can reach level 3. It's really up to how evenly they are slowed down. If Gigas are bombed twice and frozen/stalled once, they are dead. If they are only bombed once, they need to be frozen AND stalled at least once, or stalled multiple times. Row 1/4 somehow has a better Giga resistance.

Football: They are dead if they are frozen within the Gloom range. There may be exceptions: 5:54 and 6:14. All Football Zombies are harmless except those two.

Pole: Keep an eye on them when planting on column 6. When you have to stall on column 6, and they jump over, there are 3 cases:
They have been frozen: the Fumes and Glooms will usually kill them before they bite.
They have not been frozen, but are slowed down (6:45): may inflict negligible damage to the Gloom.
They are NOT slowed down at all: can significantly damage or even kill the Gloom, especially on row 1/5. Don’t hesitate. Put a Pumpkin on immediately.

Zomboni and Jack: may have a tiny chance of killing the Gloom

Digger and Balloon: of course

Sun: I lost 1650. I got lucky on wave 9 since there aren't many Gigas. If there are, use an extra Ice. For slow tempo, usually, I would lose around 1700-2000, which is barely enough for 4 Sunflowers, but on othr easier levels you can save some sun.

References:
https://www.bilibili.com/video/BV1654...
https://www.bilibili.com/video/BV1GL4...
   • Square Cobless | Day Endless  
https://www.bilibili.com/video/BV1Y84...