This achievement might take about an hour to unlock. 10 minutes of that is setup time, the rest is waiting for 250k bloons to be popped.
Load up the special mission 'Wizard Lord' and get to round 23 (easiest way to do this is to spam 1/1 dart monkeys until round 16, using the wizard lord's ability when needed, and then upgrading to 2/3). On round 23 sell all of your towers, which should leave you with about 40k to work with. Buy 3 1/3 glue gunners near the front of the map, with one of them able to reach the first corner (image of setup below). Then buy a 4/1 boomerang monkey on both the left and right exit paths. If you still have enough money, buy a 3/2 ice tower next to each boomerang monkey. This setup gives you infinite popping power, meaning it's impossible for any bloons to get past.
Image of setup:
Order of priority:
Glue gunners, Boomerang throwers, Ice towers.
If everything goes correctly, you should now have an infinite lead regrow farm. I would recommend getting a 4/X dartling gunner as soon as possible, as the ice/boomerang method is extremely laggy. If you're using the dartling gunner specialist building then you can lock the dartling gunner to the right path and play another game while you wait (the lead bloons only move along the right path).
Cheers lads