This achievement has a few steps and is awarded 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 and increases nail damage from 9 to 13.
The third requires 2000 geo and two Pale Ore and increases nail damage from 13 to 17.
The fourth requires 4000 geo and three Pale Ore and increases nail damage from 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 the nail. For this achievement, DO NOT ATTACK HIM. Just ignore him and head to Nailmaster Sheo's hut in Greenpath:
Go inside and you will find the Nailsmith here as well. Talk to him and he will thank you for not killing him, after which this achievement will unlock.
Note: you might need to have purchased the Nail Art from this Nailmaster and possibly the other two Nailmasters before the Nailsmith will be here to talk to.
If you chose to kill the Nailsmith after getting the final upgrade, that would unlock the Purity achievement but lock you out of this achievement.