Bloons TD 5

There are only 10 special missions in the game, and most of them are either easy or self explanatory, except for two (Special Agents Only and Short-Lived). If you are looking up a guide on this Achievement chances are you're stuck on one of these two, so these are the only two I'll go over.

For special agents only, I recommend unlocking pro for each and every agent first before attempting this challenge, as it makes this mission much easier. Once you have pro unlocked buy about 50 pro bloonsberry bushes and cover the entire track with them, buy about 15-20 pro Radadactyls and put them on either side of the track all the way down, buy about 10 pro bloonsday devices and layer them on the right side of the screen, buy 2 pro meerkat spy and put them at the beginning of the track, then buy about 5 of the rest (except for banana farmer, portable lake, and pontoon) and spread them out everywhere else. This should be enough agents to get you through all the rounds without thinking too hard, but if you find your towers struggling activate the bloonsday agents specials to absolutely destroy anything in their way, and since they are on the right side of the screen when the beams travel to your cursor they will hit all the bloons on their way.

Next for the short live mission, make sure you have sell price fully upgraded, meaning when you sell a tower you get a 99% return on it. When you go to play the special mission take it one round at a time and play the game how you would normally, except keep a piece of paper and a pen by you to record which tower you placed down at what round. Once you see a tower is on its 9th round, sell it before you start the 10th so you get your money back, then simply buy it again. Since you virtually lose no money from selling now you can beat this how you normally would, just be careful and keep track of your towers.

This should help you get the Mission accomplished achievement! Good luck!