If you completed the game but someone died after the landing on the Collector Ship, select someone else. On the final part where you select for the last time, do not take someone without loyalty (e.g. you lose it after you have completed their loyalty mission) as they will not survive. Instead, leave them with loyal company. For example, if you lose Tali's loyalty and take her through the final part of the mission and she dies, reload the game and leave her with everyone else. You can then get the achievement.