In the vicinity of near the Impassable Greatbridge Site of Grace is a fight between giant demon dogs. They'll start fighting the moment you walk up to them Elden Ring Runes, and from there you can just take a seat and relax. I recommend sitting on the rocks next to the wall with the damage, because you'll have a great view and will be safe from danger.
The soldiers will give you just 200 runes, but there are loads of them. Meanwhile, the dogs will provide you with 313. The soldiers will always win the fight, but you can prolong the battle by bringing the other dogs that are hanging out just north of the battlefield. You'll walk away with between 2,000 and 3,000 runes, together with Site of Grace right next to it, you can go back to the battle in just a few minutes.
Watch out for soldiers on the tower right next to you, because they can shoot you with crossbows. There's some loot on the top of the tower, however you can leave them out in your choice, since they will not be able to hit you if you're past the main fire, as is the case if you're in the rock.
The best farming spot found so far will see you going underground in a tough area however, don't worry, it's simple. TagBackTV has discovered this secret spot and you can witness the process by watching the below video.
To get to this farm site requires a bit effort. You'll need to finish Varre's quest before you can go, which involves visiting him in Liurnia's Rose Church, invading three other players cheap Elden Ring Items, and soaking an ruby in blood of a corpse you'll find in the Church of Inhibition.