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The noticeable benefit is that you get shard decreases together with the spirit farming. Essentially go to the bonfire behind the secured door in the midsts kill the 2 hollows as well as scums in the hallway just outside. Works much better if you have a strong tool or spell to send off the slimes rapidly.
You can take these out with a fireball, or drop down and eliminate them from behind. The various other 3 frogs can then be killed from behind without them attacking. Each frog goes down 241 souls, for 2,410 souls for the frogs, plus ~ 600 from the rats you killed. If you wish to farm mankind and/or shards, kill the sludges near the bonfire, as well as before you head down the ladder, go to the next space where 5 rats are collected in a corner. This approach is generally the same approach as the farming for environment-friendly titanite approach.