Finish waves 1 through 20 on any map in Infestation.

This achievement can be unlocked on any difficulty, including casual, across multiple playthroughs.

Infestation saves your progress after each wave, allowing you to quit after wave X and resume your progress later, alone or with players.

I completed the first 17 levels on Black Market at normal difficulty with a friend over XBL. Two days later, I completed waves 18, 19 and 20 by myself on casual.

The marketplace proper is a great place to make a stand. It is blue-ish and slightly more lit than some of the caves/tunnels in the rest of the level, but it offers a large square area in which to fight. The corner L-shaped buildings have only two entrances, making them easy to guard with one or two players. Destroying the centre building(s) will allow you to clear the room more easily in later rounds, as it forces the enemy AI to stay on the ground rather than camp the wall outside your stronghold.