This is the most time consuming achievement in the game, there are weapon challenges for each weapon. This includes buying a weapon and getting a certain amount of kills with them. This also requires you to play each mode and win a certain amount as well. This pretty much requires you to use equipment and get certain amount of kills and heals and multi-kills. You can do this legitimately but it is very difficult. It looks like this will have to be boosted. There is a fantastic guide here thanks to Ayx.