All weapons in the game, outside the starting Hero weapons and any downloadable or bonus weapons, are Legendary Weapons. Go to your Sanctuary and go to the weapon room to look at your weapons. If you press Y while looking at a weapon it will list 3 objectives for you to complete while using that weapon.
Find a weapon that best fits your alignment (Use weapons that require good actions if you play a good character) and look the easiest to complete. Everyone gets different weapons in their game, but you can check this link to see what all the weapons and their requirements are:http://projectego.net/fable-3/legendary-weapons
Once you complete all 3 objects for one weapon, the achievement will unlock.