This is a very rare achievement but it is not hard to get. It is pretty much a free monkey knowledge point, it takes less than 10 minutes, it needs no monkey knowledge, it needs no micro, and it is not random.
The All for One Aand One for One Achievement Guide
Start Your Game and the One Big Rule
The map we will be playing is logs on hard standard. There is one big rule here: sell before buying. When swapping towers, sell your first tower before buying the new one. If you buy the second tower and then sell the first, the run will be invalidated and you won’t get the achievement.
Winning (Part #1)
Start your game off by placing a monkey buccaneer in the bottom left corner of the pool of water in the center of the map. Begin to upgrade your monkey buccaneer to 042, also known as the monkey pirates upgrade with the crows nest upgrade. Use the MOAB Takedown ability on round 40 to insta-kill the MOAB.
Winning (Part #2)
Wait until round 49 and turn off auto start if you have it on. At the end of round 49 sell your monkey pirates and buy a 024 monkey ace, also known as the spectre upgrade with the spy plane upgrade. This tower will solo the rest of the game.
Now, wait until your spectre defeats round 80 and grab yourself your free monkey knowledge point in the achievements menu.