There are a total of 5 medals in a mission you can obtain. Obtain them all on a single play through to unlock the achievement.
This can be obtained on any difficulty.
Easiest mission for this, in my opinion is Mission 1. The medals are as follows
Mission complete - Complete mission 1
Efficiency medal - complete the mission in 16 minutes or less
Enduring Service Medal - Complete the mission without dying
Defense Breaker - Destroy all TIEs before taking out the jammers
Loyal Recruit - Destroy all Imperial Jammers before the Cruiser to call for reinforcements.
Simply set the difficulty to 'Story' and work through the objectives. Focus on the TIEs when that is an objective for Defense Breaker. Once you can attack the cruiser focus on the jammers first. Once destroyed attack the cruiser to complete the mission.
NOTE you cannot get the achievement Together, Vanguard while following this guide.