The guide provided by the team at the Mass Effect Wiki site will help you to unlock this trophy. I used this guide on both the Xbox and the PlayStation.
All credit goes to the wiki, I am only sharing my resource with fellow players who might need some help.
http://masseffect.wikia.com/wiki/Completionist_Achievement_G...To unlock this trophy you need to follow the guide linked above while making sure you ALWAYS have Garrus on your team when choosing your squad members once you have recruited him during the expose Saren mission.
You must complete 45 assignments with Garrus before this trophy will unlock. Complete the main missions as instructed, making sure to still always have Garrus on your team.
Follow the link provided and you should have no problems unlocking it.
If you want to unlock all of the 'Ally' trophies you will have to complete a total of three playthroughs using different squad members each time.