If you're going for the achievement where everyone survives the last mission, it's going to be extremely difficult to accomplish on one play through of the game alone. After the fight with Miranda and Jack, Shepard cannot have enough paragon/renegade to gain back the trust of the other (or it is just really difficult, I couldn't do it). If you really want the achievement, try playing through the game a second time. If you want to try for the achievement in one play through I think you have to choose the class that boosts your paragon/renegade. You should be given a paragon speech option in the fight between Miranda and Jack that will stop the fight and you will not lose loyalty from either. If you are even slightly renegade, I think the option is blocked to you.