Welcome to the BloonsGuide.com walkthrough for today's (December 6th, 2023) Bloons TD 6 Advanced Challenge: "Dark Days" by PhantomSpirit!
In this short guide, you'll learn how to beat the Advanced Challenge, and you'll better understand why this strategy works.
Let's get started.
Note: this solution (and its counterpart) were made possible today by Ethan Reid and Reddit users Darththorn and legomarioset. Thank you all!
The Solution
The main Tower we're going to use to solve this challenge is the Monkey Buccaneer.

Shoots a single, heavy dart from both sides of the ship. Must be placed in water.

$500
In this challenge, we'll be upgrading the Monkey Buccaneer to its Paragon form, Navarch of the Seas!
But before we do that: first, place three Monkey Buccaneers in the water and upgrade the first to 5 / 0 / 0, the second to 0 / 5 / 0, and the third to 0 / 0 / 5 to prepare for the Paragon upgrade.
Then, start the challenge and immediately use your Pirate Lord ability to hook three of the regular MOABs in.
Directly after using the Pirate Lord's ability, upgrade your Pirate Lord (0 / 5 / 0 Buccaneer) to its Paragon form, Navarch of the Seas.
It will take a while before the Navarch's ability becomes available to use. It may even be at the very last moment of the challenge!
However, as soon as the ability comes up as the BAD MOAB is about to cross the finish line, use it to instantly kill the BAD MOAB.
And that's it!
Alternate Solution
Some Reddut users have noted that the Epic Games store has yet to update BTD6 to Version 40, making it so that you're 1 gold off from purchasing the Paragon at the start of the challenge.
In this case, you can follow Reddit user legomarioset's solution:
Place a 0 / 0 / 2 Monkey Village and buy a 3 / 0 / 0 Buccaneer and a 0 / 0 / 4 Buccaneer in its radius. Then, sell the Monkey Village and upgrade the Buccaneers to 5 / 0 / 0 and 0 / 0 / 5 respectively, and buy a 0 / 5 / 0 Buccaneer. Then upgrade them to the Paragon.
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Thanks for reading, and happy popping.