This might seem hard but it's actually very easy.
As long as you or your companions (A.I or online players, it doesn't matter) don't lose all your health and need to be revived you should pop this at the end of a mission.
Keep in mind that MISSIONS are the full quest and not a subsection. The kind of mission you do it on doesn't matter (mine popped during the campaign) but missions like The Deep Below (in Winter Tundra) is a quick mission that can be farmed for the trophy in about 3 to 5 mins per run.
If you or someone else is downed return to the Quinjet and restart the mission! Restarting the checkpoint doesn't count!
The missions will have various tasks that will reduce your rating at the end if they're applicable. One star is always at risk from a downed member but the others can be anything from losing 1 star for every 25% of a capture point that AIM controls or losing 1 star every time someone you need to protect dies.