If the conditions are to be random, might as well make it so that there's a chance on spawn for it to be wieldable period, and most chances will not be able to wield it.
As in like: For every time it spawns, have a 90-98% Chance for it to always fail at being wieldable no matter what, and a 2-10% chance to always succeed at wielding regardless of conditions. That way if you see it spawn and can't seem to wield it, you know you were never meant to wield it that time, and don't have to worry about conditions that you may never figure out.
As it currently is, Dragonslayer shouldn't be needed for a Medium-Hard Badge such as Gold. It should be for something harder, like Platinum or Diamond, especially since there's no spoilers to be found unless they're outdated. And if the requirements keep changing due to people figuring it out, then eventually you will run out of easy/near-obvious conditions for it and resort to methods so cryptic, NO ONE will figure it out.