This Achievement is for enhancing a weapon to lvl 100 which means you have to enhance a weapon to lvl 10 by feeding it with items, then smelting it - which is the 2nd option at Yoshitsune- with the same weapon or an Artisan Treasure Sword that can sometimes be obtained through opening a lockbox. Depending on the level and rarity of your weapon of choice can be pretty expensive - so I advise if your only going for the achievement you can craft multiple Odachi (the first weapon you craft for Yoshitsune) as this is the cheapest way but also a very time consuming one.
WARNING: Weapons can be destroyed if the smelting fails , to prevent that you can use upgrades which looks like a single red (+10% success rate) or golden (+20%) card. You can also use some Ryu and buy the first upgrade which give another 5% success rate. Success Rate is maxed at 95%!
If your stuck and cant upgrade weapons any further you should level up Yoshitsunes Friendship Level to 20 first as you can only upgrade from 90-100 if she is at least lv 20.
Put any questions or solution problems in the comments below :)