For this achievement, you need to complete all challanges for any loadout weapon. This means only weapons you can equip, so that rules out the knife. In your "Feats of Prowess" you can find all the weapon challenges for each weapon.
For most weapons, you will have to get 1,000 kills, and 500 headshots (rifles, submachine guns, etc) OR multi-kills (explosive weapons) OR close-range kills (shotguns) depending on the weapon, so once you get a weapon unlock (which you will get plenty), you can choose a weapon you like and work with it for many, many matches. Along with the mentioned challenges, you will have to get a certain number of kills with every mod for that weapon, but these are mostly between 30-60 kills per mod.
So just keep checking what you need for the weapon you are working on, buy all the mods and complete each challenge in order to unlock this achievement.
My personal recommendation, though, would be to use the first DLC update's Falcon Blade. It only has 3 feats to complete: Buzzsaw (3 kills in 2 seconds, easily attainable in Psych warfare while the enemy team is demoralized), 750 kills (which is less than the regular 1,000), and 10,000 attacks deflected (using the right mouse button to deflect attacks, a pure defense loadout can net you loads of these), plus it has no mods. 10,000 attacks deflected may sound like a lot, but by getting shot, these will rack up pretty quickly. A nice way of getting these without jeopardizing your team is to create a defense loadout and act as a decoy. For some reason, people with katanas get shot at first, and by grabbing their attention, a teammate can take a shot at them or come up behind them. Just try to stay close to healing supplies and you won't die as much.
If the Falcon Blade isn't your kind of weapon, I would at least recommend not to try for one of the explosive weapons, due to the insane amount of multi-kills you would need.
Good luck, and enjoy.