You can find the enchantress in the caravan, press
a few times. This is where you can "master" a weapon. Despite what the achievement description implies, you don't need to max every status effect for a weapon, you just need to get one attribute to 100%.
You need 10500 jewels to max an attribute from 0%. You can use a weapon that comes with an attribute with some progress to make it a bit easier. After you've maxed an attribute, stay at the enchantress screen and let the achievement pop. Dashboard and reload the game to save your jewels, unless you want to keep the attribute maxed.