Gain the loyalty of the mercenary
This is a missable achievement.
First time you talk to Zaeed, you will get his loyalty mission. In order to get the achievement, choose to go after Vido instead of saving the refinery workers. However, if your Paragon/Renegade is high enough, you have access to the dialogue options which would allow you can save the workers (paragon route) and still have Zaeed become loyal to you.