***This has changed since I played it last, lots of valuable info in the comments for the completion of this achievement.***
Okay Traits are first unlocked at Rank 15
This means grind your way though multiplayer till then.
Once you hit 15, buy the card; this will be "Bodyguard", "Scout" or "Survivalist" each one costing 7,000 credits and equip it to your character.
Leveling the card to level 3 has to be done in one life and you need a certain amount of kills to level it each time.
Personally I found that using the "Scout" trait was best as it allowed me to move around the map sprinting without being seen on the radar.
3 kills to level 2
Further 3 kills to level 3
I recommend doing this on blast as the objective is to get kills and that's what your trying to do. Walker assault and supremacy were recommended by A Krayt Dragon and Black Man 8698 for the large amounts of players and longer time limits.
> Rank 15 minimum
> 7,000 credits minimum
> "Scout" (recommended)
> Blast or walker assault (recommended)