Blacksmith of the Sand Kingdom

There's no point grinding for this until the very end of the game.

By the time the game ends, you'll have a lot of random runes. Simply go into the modify part of the Workshop menu, then just randomly add runes into any item you like. If you choose to do it in a low rank weapon (basic knife for example), create as many of them as you can, they will always appear at the top of the list. When selecting the runes, press to filter out the runes you can't equip and you can equip two runes in a weapon every 5 seconds. Doesn't take too long after that once you get the hang of it.