Welcome to the BloonsGuide.com walkthrough for today's (December 11th, 2023) Bloons TD 6 Advanced Challenge: "4k OP" by Lolcans!
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.
The Solution
The Tower we're going to use to solve this challenge is the Glue Gunner.

Shoots a blob of sticky glue that slows Bloons down by 50%

$245
The Challenge
To start the challenge, place a Glue Gunner within the first full loop of the map and upgrade it to 3 / 1 / 0, or Bloon Dissolver in the top path:
And that should be enough to take this challenge down!
Why This Works
There aren't many hidden mechanics or secret tips for this challenge. But the Glue Gunner is great at taking out the Bloon types thrown at us during this challenge.
Rainbow Bloons are fast. The Glue Gunner's slow effect mixed with the Acid from Bloon Dissolver slows the Rainbows down enough to pop all their layers before the end of the map.
Furthermore, Bloon Dissolver does extra damage to Ceramic Bloons. Per the Wiki:
"Corrosion rate increased to 1 damage per 0.5s or 2 damage per 0.5s to Ceramics, glue gains +1 pierce."
Finally, since Bloon Dissolver melts two layers of Bloon every second, it's perfect for dealing with multi-layer Bloons.
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Thanks for reading, and happy popping.