Stray Blade

There are a total of 32 weapons in this game. Two weapons are just found lying on the ground right at the start of the game and do not count towards this. Those two weapons are Rusty Sword and Wooden Club. For the remainder, 22 of the weapons are human style weapons and these blueprints are gained from killing human enemies, generally after leveling up and killing the next human enemy.

The remainder 8 weapons are Ellesine weapons which are in set locations around the world to find the blueprints. Check out my guide here for those locations.

Once you find all the blueprints and craft every weapon, this will unlock.

It's not a guide, just a warning!!

Another game that has several achievements with bugs, perhaps after the post DLC update. In my case, this and others did not open for me.