Updated 04-24-2016 11:46 AM
To unlock this achievement you simply need to complete all the challenges associated with any one weapon in the game. There are 10 primary weapons, 4 anti-Titan weapons, and 3 sidearm weapons to choose from. The weapon challenges all have 5 tiers of challenges that must be completed for the achievement. You can check your status by following this path from the main lobby:
Lobby / Challenges / Pilot Anti-Personnel Weapons
Lobby / Challenges / Pilot Anti-Titan Weapons
Lobby / Challenges / Pilot Sidearms
The achievement is not very hard to complete. Pick a weapon and stick with it. The achievement will unlock well before you hit level 50, but if you want to complete this achievement quickly you can pick the Mag Launcher or the Archer Heavy Rocket. They both only have one challenge associated with it. Complete the 5 tiers and the achievement is yours.