STAR WARS: Battlefront Classic Collection

To obtain this achievement you must earn 20+ medals. This can be achieved offline playing against bots. You can check how many medals you have at anytime by doing the following. Select profile>Main menu>Options>Career stats.

The 9 different medals are

Point medals:
Endurance - Earn 12 points without dying
Guardian - Earn 24 points without dying
War Hero - Earn 36 points without dying

Kill medals:
Gunslinger - Get 6 kills without dying with the blaster pistol
Frenzy - Get 12 kills without dying with the blaster rifle
Marksman - Get 6 headshot kills without dying with a sniper rifle
Regulator - Get 8 kills without dying with the shotgun

Vehicle medals:
Technician - Slice an enemy vehicle with a fusion cutter (an enemy must be inside)
Demolition - 4 rocket launcher critical hits on a vehicle within 1 life

I suggest using the infantry class while going for the point medals along with the frenzy and gunslinger medals.

The following is how many medals you need for each rank and the command ability reward.

Private: 0 medals (Lead 1 unit)
Sergeant: 20 Medals (Lead 2 units)
Captain: 100 Medals (Lead 3 units)
General: 300 Medals (Lead 4 units)

Reaching Sergeant rank requires earning 20 medals

My understanding this is only in Battlefront 2

Easiest medal to earn is get 12 points in a single life