Not sure where you are in the game, but if you travel to Bestial Sanctum, save at the grace inside the building, and basically there are these black grim reaper looking dudes right off to the left and right of the entrance. I usually go left, as they are more spaced out. They are tough because they are a higher level than when you're supposed to be there, but they have low HP, and you can backstab them and take them out pretty quickly. They give between 1,000 and 1,200 runes each. Kill 7 or 8 of them, go back to the grace, rest, level up, and then they will all respawn and you can kill them all again. I can usually pick up 300,000 or so runes in an hour, once you know how to kill them. Just avoid their spell attack (it'll kill you instantly, but is pretty easy to avoid), and their charged jumping swing attack (same thing, insta kill). You can probably even kill 15 or more before heading back to the grace, but just be careful because you obviously don't want to die with 50,000 or more runes on you and then get killed trying to get back to your corpse. I usually get enough runes to level up, then go back to the grace and spend them, just so if I do die, I don't have a shit ton of runes on me and then have to sweat out a corpse run.
If you don't know how to get to the Bestial Sanctum portal, it's right behind the Third Church of Marika, behind some bushes. I think it's north behind the church.
