Completed all challenges for a single weapon

For this achievement, you will need to complete every challenge set for at least one weapon. Since I'd suggest unlocking all attachments for the guns during your first Generation, you'll likely be close to this for a few guns by the time you unlock that achievement. Plus, the Generations after the first require gun-specific challenges to be completed before Regenerating and have accelerated leveling, so you may not have the luxury of sticking with one gun long enough in any of them.

The Archer Heavy Rocket and Mag Launcher anti-Titan weapons are by far the easiest weapons to use to unlock this achievement. Both only have one challenge:

-Kill 10 Titans

The R-101C Carbine is probably the most user-friendly primary gun in the game, and the challenges are fairly simple as well. This would be the next easiest gun, though it will take much longer than the Archer or Mag:

-Kill 75 Pilots
-Kill 100 Spectres
-Kill 200 Grunts
-Kill 200 Enemies
-Score 75 Headshots
-Use for 3 hours total