All for One and One for One

 is an achievement in BTD6, first introduced on Version 13.0. It can be achieved by achieving Hard Difficulty using only one tower at a time. It rewards 1 Monkey Knowledge Point.

Tips

 * A few Advanced Challenges on Hard that only allow one tower will instantly grant this achievement upon completion of the challenge.
 * Like other tricky challenges, always attempt on Logs, due to the map's long track that loops over itself multiple times.

Strategies
Feel free to add your strategy for earning this achievement here


 * In 22.0, a 0-5-2 Pirate Lord can solo up until round 63 (with a small leak at 46), which then can be swapped for 0-2-5 Prince of Darkness. On round 80 buy Pirate Lord again and use the ability. Tested on Cubism.
 * Retested on Standard Hard Logs on v23.2.3593; no leaks, achievement completed.
 * Retested on Standard Hard Logs on v27.1.4229; changed 0-2-5 Prince of Darkness into a 0-5-2 M.A.D no leaks, achievement completed.
 * Retested on Standard Hard Logs on v27.3.4285; changed 0-2-5 Prince of Darkness into 5-2-0 Apache Prime no leaks, achievement completed.
 * Retested on Standard Hard Logs on v28.3.4683; delayed the placement of 0-2-5 Prince of Darkness until round 76 no leaks, achievement completed.
 * One strategy is to start with a 0-2-4 Favored Trades for early game (prioritizing 0-2-0, then 0-2-2, then 0-2-4), then switch to a 5-2-0 Apache Prime for mid-late game.
 * Some bloons may leak using the Buccaneer, but not enough to cause a game over (Mana Shield MK is recommended). The Favored Trades provides extra income to help afford the Apache Prime sooner.
 * As of 22.0, the Apache Dartship costs too much to be available by round 40, but a 3-0-0 Heli Pilot can pop the MOAB depending on the map.
 * One dangerous round is 63, but the Apache Dartship is usually capable of dealing with the Ceramics.
 * Retested on 2 Player Co-Op Standard Hard Logs on v30.1.4987 and on Solo Standard Hard Logs on v30.2.5032:
 * Sell Monkey Buccaneer after round 39.
 * Purchase 3-2-0 Heli Pilot.
 * Upgrade to 4-2-0 Apache Dartship and 5-2-0 Apache Prime when affordable. Micro isn't necessary.
 * One strategy is to place Sauda in the center to the right of the pool in Resort, leveling her up as soon as possible, and using Sauda's abilities at crossings in the track to deal with Lead Bloons. As of version 26.0, this can be completed with minimal leakage.
 * Retested on v30.1.4987; this strategy is no longer viable.

Trivia

 * 28th/29th October 2019's Advanced Challenge can reward this achievement. This was likely unintentional.
 * Any other Advanced Challenge On Hard Difficulty that only allows one tower can also reward this achievement, such as 14th/15th December 2019's Advanced Challenge.
 * This achievement used to be a hidden achievement until an update.