For this secret you will need the "Ice tower" and you have to have the Arctic wind upgrade (If you don't have that upgrade keep reading I have useful info.)
This secret works best with a 0-4-2/1 Ice tower because of the big range. A 0-4-0 or 0-3-2/1/0 would also work but without the extra range that Larger radius and Snowstorm gives placing towers are a lot harder and in some cases makes it impossible to place a Helicopter monkey.
This is the introduction to this guide in the next section I will tell fast and easy ways for getting XP with the Ice tower.

Stay cool :) --Dev7z

Getting XP for the Ice tower is not easy or it is easy but it is a really slow process.
Thats why I only have 49k XP. The fastest why to grind any Tower is to play a medium game and only use the tower that you want to grind XP with (With the exception of Village and sometimes Banana farm of course.)
If you use only 1 tower + village if needed in early game and in late game by round 100 that tower should get lots of XP but the problem with Ice tower Glue gunner and Bomb is that they can't poop all of the bloon types in early game; Ice tower can't pop whites, Glue gunner can't pop any bloons without upgrades, and bomb can't pop black bloons. Keep in mind none of them can see camo bloons.
What about an MIB you ask?
Good question... and the good answer is unless you use lot's of cash drops (Which I don't recommend.) you won't afford a tower that costs 12k in early game.

The way that I grinded XP with ice was, to go to a hard game and simply grind XP on early game. That is the fastest and easiest way that I know but I'm not a professional in Bloons TD 6, there might be better ways.

Stay cool :) --Dev7z

It's easy just place a 0-4-2 Ice tower (0-4-1, 0-4-0, 0-3-0, 0-3-2 and etc. also work but trust me 0-4-2 is the best.)
Make sure the Ice towers range reaches the water, if you have done everything right you should be able to place any tower in the water. If you have any problems (which I doubt) you can message me for help, since I don't expect anyone to read this I don't think I am going to get spammed :)

Stay cool --Dev7z

The way that Ninja Kiwi intended for this was that the Ice tower freezes the water for other monkeys.

You made it! You read a text in a gaming platform instead of playing the actual game.
Just joking thank you for reading and I hope I helped you :)

If you need help with anything about Bloons TD 6 or if you want another guide feel free to contact me :)

Stay cool and use Snowstorm --Dev7z