This is relatively straightforward - in Mission 5, you need to obtain the "Marksman" medal.
Marksman - Prevented any missile from damaging the GR-75s.Tips *Spoiler Alert*At the beginning of the mission, when you travel stealthily through the debri field, Cpt Amos will show up with his Star Destroyer to attack the convoy you are tracking. Best thing to do here is to position yourself alongside the Destroyer, aiming toward the convoy. Simply cycle through the targets with
and shoot them down - should only take a few shots per missile (remember you can put power into your lasers to prevent them from overheating with
). If you basically park alongside the Destroyer you shouldn't even get shot at, just takes some aiming really.
Edit - This achievement will unlock at the end of the mission (as that is when you obtain the medals) - thanks to ReaperShadow666 for pointing this out!