This achievement is story related, so you can't miss it. After a chase through Illium, you end up fighting Vasir, an Asari spectre. She is quite tough, and moves around quite quickly. At several points during the fight, she will leave, and you'll have to deal with various drones and enemies. Once she comes back, just repeat what you did before. Before the battle even begins, make sure that you take the time and invest all of Liara's points into her skills, especially if you are a higher level. Her biotics will make taking care of Vasir much easier. When she has been defeated, the achievement will pop.