The Proof of a Concord Kept covenant item is one of the most difficult items to PvE farm in Dark Souls III.
I generally don’t use consumable items when farming more because you waste a lot of time going to buy them, but this is one item where it’s worth it to buy max Rusted or Gold Rusted Coins and use one about every time you do a run.
Here’s a way to be able to use a Hollow +10 weapon as well as the usual Crystal Sage’s Rapier, Symbol of Avarice, and the Covetous Gold Serpent Ring +3. For it to be effective, you have to have both the Crystal Sage’s Rapier and your +10 Hollow Weapon in your Right & Left Active Hand Slots when each Anor Londo Silver Knight dies.
This means you can’t one-shot them with your spell, or even finish them with your spell.
For this run, I used the following:
Rings – The 2 Sorcery Boosting rings, the Magic Clutch Ring, and the Covetous Gold Serpent Ring +3.
Spells: Crystal Soul Arrow, Homeward, & Sacred Oath
That’s what it takes my current build to bring the knights as close to death as possible without one-shotting them, set yours up to do the same, or maybe use Homing Souls or a combo or whatever works for your build.
I used Hollow Brigand Twindaggers +10 here , as well as the Church Guardian Shiv.
After hitting them with the spell, switch from your catalyst to your Hollow +10 weapon, then either hit them with the weapon, or throw a few Shivs, either way you’ll get the Discovery/Luck Bonus for holding your +10 Hollow weapon.