At a workbench you need to create 10 of the alchemy recipies.
If you're playing as a Technician (As I gave tips for in the first Storyline trophy)
You'll have the Science skills. When you hit level 7, put your talent point in to Science level 2. This allows you to create Ammo.
Go to a merchant and buy the ingrediants to make ten of them and then return to the workbench and create ten bullets for the trophy to unlock.