This achievement isn’t too hard at all. From the main menu, select “Single Player” then “Survival”. From there, you’ll just want to pick a map, start the battle, and wait until you lose. You can just leave your controller, or spawn your hero and damage your own gate to speed up the process. Do this for all 8 maps and you will unlock the achievement, which should take a total of 15 minutes.
Note: You can use your ballista to try and kill all but 1 enemy. This way you will earn more gold, and will lose just as fast.