At the bridge where the red dragon is sitting, run across the bridge and he will spit fire at you. Go down the stairs on the right side of the bridge and kick the ladder down. There is a bonfire at the bottom of the ladder. Rest at the bonfire to and make the enemies spawn back in place. After that, go back up the ladder, out the door that leads to the path under the dragon, and stay near the wall on the left so the enemy doesn't see you. Shoot the dragon's tail and run back quickly up the stairs about half way or more. The dragon will spit fire, killing all the undead. That gives you 300 souls. Go back down the ladder and, before you rest at the bonfire, kill the undead out the door and get around another 300 souls. Repeat until you have the desired amount of souls.