Ready or Not

There are 12 officer traits you will need to unlock.

Armorer, Breacher, Intimidator
Kicker, Moral Officer, Negotiator
Nutricionist, Pacifier, Paramedic
Riot Control, S.B.A.G.S., Veteran

You cannot choose which trait is given to an officer, it's random.

The traits are unlocked upon successfully completing missions with the officer alive.

You only have 1 trait per officer.

Best way to "farm" the achievement would be having all 12 slots unlocked, keep track which traits have been unlocked and fire (Y) officers that already have their trait unlock. Only 1 officer can be fired every time you finish a mission.

Replay the first mission on standard to minimize the risk of losing the officer, you will average 4 - 5 suspects per mission. Once you learn the level it should take you around 3-5 minutes. I advice using non lethal weapons (VPL-25 or Bean Bag Shotgun) to keep using the officer without affecting his mental state. They can resign if left untreated.

Only 1 officer can unlock their trait after finishing a level, and it's random. So only have officers with the trait you are missing locked.