This trophy / achievement will unlock as soon as you fully upgrade a complete armor set to level 9. This means that you will need to upgrade each piece of a Chest, Arm, and Waist from the same set. For example, if you upgrade each of the Nidavellir’s Finest Plackart, Nidavellir’s Finest Arm Guards, and Nidavellir’s Finest Waist Guard to level 9, that will count as fully upgrading one armor set. The requirements for fully upgrading each piece from a particular set are sometimes slightly different, but there are similarities across the board in terms of the type of metal required as you progress to new levels, as you can see on the Nidavellir Finest Armor Set page.