Wolnir
Maximum souls: 327,175
Starting from the Abyss Watchers bonfire, head down the stairs and keep to your right. You will find a shortcut that leads on the way to the Catacombs of Carthus bonfire. Don't light the bonfire. Instead head to the boss arena of Wolnir. The boss is easy to cheese. Equip Pestilent Mist, Poison Mist, and Toxic Mist. After touching the goblet and triggering the fight, stay to your right and walk to Wolnir's hand. Don't hit the boss. If you're equipping any pieces of the Armor of Thorns, be careful not to roll into the boss. When you're close enough to the hand, cast Pestilent Mist. Then lock on to the hand and cast Poison and Toxic Mists on Wolnir's hand until he is toxic and poisoned. Continue casting Pestilent Mist until the boss is defeated. Refer to this video for the cheese strategy:
