During the Brother 2 quest, very near the end, Abby will request you get 2 keys to provide her access to Nazi computers. You will be on the roof of the building, and a mech will rise out of the center. You must kill this mech to get the achievement. It is possible to complete the mission without killing the mech. The mech is weak to heavy weapons that are at 3 o'clock on the weapon wheel.
If you missed the achievement, talk to Abby in the catacombs to replay the mission. You can play it on easy.