For this achievement, you'll want a main character that can equip all items, so pick either regular Paladin or regular Fighter.
Pick up the Helm of Balduran from the very first level of the game. Buy the Sword of Balduran, Shield of Balduran, and Plate of Balduran from Deidre in the Adventurer Mart in Athkatla. (Very expensive items!)
Once you have all the items, equip them all onto your main character, and the achievement should unlock! If it doesn't, try going to another area or sleeping while having all four pieces equipped.
Note 1: Many classes can't equip all the items, so make sure to pick a class that can! (As mentioned, regular Figher or Paladin works well for this.)
Note 2: This only works with your main character! Equipping Minsc or anyone else with this won't do you any good.