#Edit1 - thank you OldMateClown, looks like since patch (And DLC = Echoing Void) there is a big problem with this achievement so for now the best thing you can do is to keep your weapon in your inventory and wait for the game developers to take care of it
#Edit2 - Since last update = Cloudy Climb, you are able to get the achievement there. You need to fully enchant your weapon (all 3 slots) and gild your weapon in one of the merchants (I believe this one is on 19 floor). I can not recommend if this 100% works, but I got many DMs about this so I decieded to put it here :) [Achievement should pop after you defeat final boss]
In order to get this achievement you need to play ancient hunt missions. During the walktrough you will find some dungeons (Not portals! These look more like the stone gate) after you enter them there is the chance you found like gold vault (A place where 2-3 gold chests are) or the boss area, this one interest you the most. You have to find the switch (It is almost always on the ground) and press the button to activate it, after that you have to defeat the boss. After you kill him, he gives you the item and now this is serious RNG! You have to find a weapon it means it has to be a bow, crossbow, sword etc. if you got an armor IT WILL NOT COUNT!
If you found a weapon, check the rarity it has to be UNIQUE (not rare or common). Did you get the unique? Do not celebrate too much because the last one is enchants which weapons have, the weapon has to have 3 slots of enchants, and you have to upgrade each of them to III enchant level.
Far as I know it's not matter if you have 3 slots and only 1 enchant on each because it requires to have choosen 3 enchants and upgraded to the maximum.
Also, if you have some luck you can try to buy the item in Piglin Merchant store, and I reccommend to check it beacuse it may save you a lot of time.
And do not forget to equip it! Because this is the last step
As you can see, you might do it on your first rush, or after 200+ rushes