You need to have found and equipped a complete set of gear to your Head, Chest, Arms, Legs and Feet. Weapons, rings, and amulets do not count towards this achievement.
Playing as Bragi Axebiter you can wear Giant Slayer sets or Daemon Slayer sets.
A set has a place name e.g, Kazak-A-Karak and a type e.g, Giant Slayer.
If you equip all Kazak-A-Karak gear which is a mix of Giant and Daemon Slayer this will not work.
The quality of the equipment does not matter, I got the achievement with mostly blue items.