Q:
Legendary Weapon Snafu
Weaponsmith can use them in recipes, but can not craft them.
https://wiki.guildwars2.com/wiki/Elonian_Leather_Square
https://wiki.guildwars2.com/wiki/Elonian_Leather_Square#Weaponsmith
Good luck.
Edit: Here’s a list of the things you will need so you can start preparing – https://wiki.guildwars2.com/index.php?title=Special:RunQuery/Base_ingredients_query&Base_ingredients%5Bitem%5D=The%20Juggernaut&Base_ingredients%5Bid%5D=3069001&Base_ingredients%5Bdiscipline%5D=Mystic+forge&Base_ingredients%5Bquantity%5D=1&wpRunQuery=true
(edited by Inculpatus cedo.9234)
It has always been a requirement to craft in multiple disciplines for legendary weapons (For example, the Gift of the Juggernaut, will require you craft a Gift of Metal (Weaponsmith) and a Vial of Quicksilver (Jeweler).)
Crafting precursers added more onto the workload.