So this achievement must be done on the Luftrousers 3 game. It carries over multiple playthroughs, so it's not super hard to get, but I found a method for getting it quickly in one run for my fellow grinders. The achievement requires getting 10,000 coins (cumulative)

I did this on Wave 9, but I think it should be able to be done on any later game wave with aliens spawning.

Basically you can get as many upgrades as possible beforehand and then leave 1 Destroyer battleship alive. One alien should respawn repeatedly from the top screen, dropping about 30-50 coins per kill. If you've gotten all the upgrades at this point, you should be in no danger in dying to the enemy, and it's just a matter of time before you get enough coins for the achievement. I had around 5,000 coins when the achievement popped. You can chill around the Aircraft Carrier or around the Shop so the alien is the only thing endangering you on the screen, the Destroyer battleship doesn't need to be actually visible. Also, after this, I had all the upgrades and didn't die after idling several minutes on wave 9, so it could conceivably work as a very slow infinite score strategy on wave 9 or 10.

Have fun and happy hunting!