Sorry if this is a bit late, but I just found a method (on YouTube) that nets about 2000 souls for 5 minutes of work.
Go to the Ashlands, on the north end, there is a pit with water on the bottom. You need to have Ruin, at least, in order to do this.
When you first come into the Ashlands, there are four horsemen riding on blue horses. Every time you jump down this pit, two more horses spawn. You can repeat this to spawn around 16-18 horsemen (safely) and then slay them all at once, earning, on average 2000 souls.
You can spawn more, but doing that may freeze your game, as even 16 causes it to be kind of buggy.
I made 16000 in the course of 30 minutes. --- He who fights with monsters might take care lest he thereby become a monster. The only future peace has to offer is war.
That seems to work best. I've also tried grinding on the spiders in the Iron Canopy, but it was slower and a lot more tedious.
Also, while fighting the demon horsemen, I saw a noticeable difference by having Death's Blessing equipped on Chaoseater. It increases the souls they drop by a fair amount. --- Religion is like spaghetti: either stiff and fragile, or wet and limp.