I played on Town Meeting in Last Stand mode, and after Wave 3, there was an icon on screen that simply said badge. When I got to the location, there was a large gold round badge. Just collect it, and the achievement will unlock at the end of the game on stats screen.
I died on Wave 7 and never found another, so not sure if you can earn more than 1 per game. If you can't then you will need 200 games of Last Stand to get the other achievement. Bots don't seem to be as efficient on Last Stand as they are in story mode.