A bit late, but since there's a lot of questions in the comments : *You must collect all weapons with one character only, doesn't matter what your other characters have collected. *Moonlight Butterfly's soul, Queelag's soul, Iron Golem's soul and Gwyndolin's soul are needed twice each, and Sif's soul is needed 3 time, so it takes at least 3 runs to get all of the boss weapons alone. *If a player drops a boss soul to you, you can craft the boss weapon yourselfand it will still count for the achievement. Having directly the boss weapon dropped to you doesn't seem to work, it's probably safer to craft it yourself. *From what I gathered, same goes for non boss weapon, it seems you have to collect everything yourself and not have it given to you by another player. *It seems you can get rid of any weapon needed and still get the achievement, as you only have to have possessed any weapon needed at least once on the same character (I wouldn't just to be safe and to keep count of what you have already collected) *None of the DLC weapons are needed for the achievement (so no Abyss Greatsword, Dark Silver Tracer, Gold Tracer, Obsidian Greatsword, Stone Greataxe, Four-pronged Plow, Guardian Tail, Gough's Greatbow and Manus Catalyst are needed) *Demon's Catalyst and Izalith Catalysts don't seem to be needed. *Apparently some people got the achievement by having only one of the Artorias Greatsword (cursed/non cursed) but the vast majority of players have been needing both, idk why. Please feel free to add useful tips or to correct me if I'm mistaken on something.