Welcome to the BloonsGuide.com walkthrough for today's (December 3rd, 2023) Bloons TD 6 Advanced Challenge: "Pretty Easy" by Chartreuse!
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 main 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%.

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But that's not the only tower we'll be using. Wait and see!
To start the challenge, place a Glue Gunner anywhere on the map and upgrade it to 0 / 5 / 0, or Glue Storm in the middle path.
Then start the challenge. As soon as the Glue Storm ability is available to use, use it.
Immediately after using Glue Storm, sell the Glue Gunner and place a Spike Factory as close as you can to the dam and ahead of the Camo Lead Bloon, then upgrade it to 0 / 0 / 4.
The Spike Factory will then be able to pop the Camo Lead Bloon and the challenge will be complete!
Why does this work?
The reason this works is sneaky, and one you may not know of already!
Glue Strike
The Glue Gunner's 4th skill in the middle path, Glue Strike, actually temporarily stops Lead properties while Bloons are affected by the ability's glue. Moreover, it also makes all Bloons more vulnerable (+2 damage).
Spike Factory
Spike Factories have Camo Bloon detection by default.
When placed near the dam, the spike factory can also throw its spikes onto the track where other towers can't reach. When combined with Glue Strike to remove the Camo Lead Bloon's "Lead" properties and make it vulnerable to more damage, the Spike Factory is able to take out the Camo Lead.
Are there other solutions?
You can actually beat this challenge by only upgrading the Glue Gunner to 0 / 4 / 0 and using the Glue Strike ability, but the extra slow time / affected Bloon duration provided by Glue Storm makes the challenge a bit easier.
You can also defeat the challenge with a 0 / 2 / 2 Spike Factory after glueing the Camo Lead.
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Thanks for reading, and happy popping.