This is a pretty rare drop from the Ring Knights.
By Far the fastest way to farm these is Ringed Knight with the Dual Ringed Knight Greatswords.
Here’s my method for farming these guys using the Hollow +10 and a Crystal Rapier (Along with Symbol of Avarice & the Covetous Gold Serpent Ring +3)
You just tinker around with all your Spells, Buffs, Rings etc until you get it dialed in for this one enemy. Remember the Dual Greatswords Ringed Knight has way more HP than the rest of them. But just try combinations of Spells, Miracles, Pyromancies, Dark Casts, Staves, Talismans, Chimes, Pyromany Catalysts, Casting Weapons etc until you get him down to where a few shivs can finish him off. Then you can start an efficient farm.
After messing around for a while I finally used the Fire Clutch Ring (which out-buffs the Great Swamp Ring, 12% vs 15%), the Witch’s Ring, the Lingering Dragoncrest Ring (to remain hidden long enough) and the CGSR+3. Then I attuned the Chaos Bed Vestiges Pyromancy, the Homeward Miracle, & the Hidden Body Sorcery.
With this build it took 4 casts of CBV to bring him to within one shiv of death.
Notes & Tips:
1. After using a Rusted Coin every time I began to think maybe I should wait for the other one to expire before using one, as the drop rate was so low. But then I thought this same thing in DS2 and it really didn’t seem to be the case. I probably only did about 19-20 runs though, so that’s about a 5% drop rate so I guess that’s really not bad.
2. Use your Coin when you first get on the elevator, then you’ll have time to adjust your item positions before getting off smoothly.
3. Get as close to him as you feel comfortable before casting Hidden Body, then Power Within, then wait until your Stamina Recovers enough for 2 hits before casting.
4. After the first 2 hits, step back a bit and let your stamina bar fill enough for 2 more hits before trying to throw another one.
5. Use Saint-tree Bellvine for a faster cast home.
6. When you’re out of Coins, rest at the bonfire for them to be refilled from your storage.
7. There are more ringed nights out in the Abyssal Swamp on the sunken building’s rooftop, but you’ll also have to deal with potential Judicator Summons, an easy gank from 4 Ringed Knights, as well as the overly-hostile and back-handed Locust Preacher. You MAY get a clean run now and again, but more often than not you’re not making it out without taking quite along time to target them all one by one without being noticed by anyone else, making it far less fruitful than this lone Dual Greatsword Ringed Knight.
8. Leaving from this Bonfire is faster than approaching him from behind via the Boss Room, and the Elevator Time lets you get things in order as well.