To unlock the Assault Artisan trophy in Anthem, you will need to complete three of the Assault Rifle challenges which can be found by opening the Menu > Challenges > Weapons > Assault Rifle
You will find four challenges available -- one for each of the game's three assault rifles, and a final one for completing all three of the assault rifle challenges.
The three challenges you will need to complete are 50 kills with the following three weapons:
Defender
Hammerhead
Warden