This is a Multiplayer related achievement associated with having all weapons unlocked. You have to be a minimum level of 40 to have the weapons available to you. As NutriWhip pointed out, the achievement unlocks once all weapons are available, you do not need to purchase them for the achievement
I've got a friend who has already done this and I'm on my way to getting it.
If you downvote please explain why. I'm still somewhat new to solution writing.