You will need to not only find, but also use, all 101 weapon blueprints. Most of the blueprints will be in a location where you will have access to the necessary materials to create them; so it's a great idea to craft the item when you find the blueprint. You can also use the weapon locker to get a hold of any weapons that you have picked up previously, making the crafting aspect way easier than this tedious achievement makes it seem. Instead of listing all of the combo weapons here, simply go to your in-game menu and access the "Blueprints" screen for a list of all the combo weapons you will need to make.