To unlock this achievement, you will need to purchase every attachment for one weapon class. The best weapon to obtain this achievement with is the shotgun as you only need to purchase 4 attachments to fully upgrade it. You can purchase weapons upgrades from the main menu by going to 'Single Player', 'Agent Profile', then Upgrade shop, or alternatively, you can access the upgrade shop while playing a mission by pressing and then selecting 'Upgrade Shop'. You will need a total of 6040xp to be able to purchase all 4 of the shotgun upgrades.
You earn xp by completing objectives and challenges during missions.