You obtain this achievement for completing all of Charlie Cole's missions. These missions are horde-like, in that you eliminate incoming waves of enemies. There are 4 missions, with 20 waves each.
To make your life easier though, you only have to complete the first five waves for every mission and extract. I did this solo on three of the missions and unlocked the achievement. The other mission I had already beat all 20 waves. It'll probably take less than a hour to do all four.