You can get this achievement on the map
Candy Falls. There are monkeys hidden behind objects in this level that you'll need to click in a certain order to be able to obtain this achievement.
Rather than attempting to explain this order, I'll just link you to this image I found.
https://static.wikia.nocookie.net/b__/images/3/37/CornersOom...There's a limit to how much time you can spend between each monkey (your time being up or you clicking on the wrong spot will result in a buzzer-like sound) and if done right you should obtain this achievement upon the level's completion.
Edit: Thanks to RuneSlayer4421 for pointing out that the link doesn't work and providing a new link.
New link: https://bloons.fandom.com/wiki/Small_Monkey_Towers_Option