I think the achievements need to be setup so everyone earns them over time. Have them tied to stupid things like kills in one game, or 2 or 3 kills in a row with one weapon, something that you can do while you play the game however you like.
Why have the weapons "earned" though? Why not just give the weapons and then give the achievements? This way you can still have weird quirky achievements that a player isn't going to normally get and when someone does get it, they feel special because not many other people have it. On top of that no one has to grind or "cheat" by going onto empty servers, using achievement servers, or sabotaging their team. Everyone has the weapons and the people that like the achievements can go get them.
What the grind monkeys can't see is that by linking the weapons and achievements both are cheapened. If it wasn't a problem why has valve nerfed some of the achievement weapons and reduced the amount of achievements required to unlock the weapons? They just aren't brave enough to take the final step.