This achievement can be earned after having the Nailsmith in the City of Tears fully upgrade your nail. He can be found here:
A fully upgraded nail requires four upgrades:
The first requires 250 geo and increases nail damage from 5 to 9
The second requires 800 geo and one Pale Ore 9 to 13
The third requires 2000 geo and two Pale Ore 13 to 17
The fourth requires 4000 geo and three Pale Ore 17 to 21
There are exactly six Pale Ore
in the game so you must acquire all of them in order to complete the upgrades. Three are found in the world, one is given by the Seer after collecting 300 essence with the Dreamnail, one is given by the Grubfather after rescuing 31 grubs, and one is awarded for completing the Trial of the Conqueror in the Colosseum of Fools.
After the final upgrade, the Nailsmith is so moved by his creation that he has to step outside. Talk to him outside and he will ask to be killed by his perfect cration:
Attack him with the nail and the achievement will unlock.
Note that this achievement is in conflict with the Happy Couple achievement and only one can be earned on a given save file.