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Create 58 unique Combo Weapons.

Online required. There are 51 combo weapons in the single player game, meaning you have to make the other 7 in multiplayer. (Technically, you can make them in single player, but you have to unlock the blueprints in multiplayer first.) As noted elsewhere, you get multiplayer blueprints by completing trials. As for single player, you will earn some blueprints as you progress throughout the story, and others have to be found. You can buy maps that lead you to them as you level up safe houses.