To get this achievement, you need to equip yourself with six maxed-out items of armor and weapons from a single Witcher school. This means Grandmaster quality, only obtainable in the DLC, and a minimum of level 40 in order to be able to equip them.
You can obtain the quest locations of all five sets of armor from the grandmaster smith in Beauclair. The easiest way to find his shop is to head north from the fast travel signpost of the Metinna Gate. You can also receive a quest notice that will mark his location from a notice board.
This achievement requires you to find all six diagrams for at least one set of armor: Feline, Griffin, Ursine, Wolf, or Manticore, and then fully upgrade that set. For everything but Manticore, it means you'll have to already have acquired all of the lower level diagrams for one of the other sets in the main game and built the armor up to master quality.
Manticore has the advantage of only having the six diagrams in this DLC and you can go straight from nothing to full in a single construction (note: the Manticore armor and weapons are not actually listed as "Grandmaster" quality in the descriptions or names, but they are considered such by the game and can be used for the achievement). I also thought the components needed to construct the Manticore were easier to obtain/cheaper to buy than those necessary to upgrade the master quality items from the other sets to grandmaster quality, so this makes the Manticore set both easier and cheaper to build to get the achievement.
However, in terms of finding the diagrams, the Manticore set requires the most travel: 4 different locations zig-zaging across the map. The Feline are spread among three different spots. The Ursine and Griffin are both in two different locations; the second Griffin location is one of the Hansen ("Super-bandit") lairs that are more work to clear out. The Wolf are all at a single location, so require less travel and effort than any of the other sets to collect.