User blog comment:Mathmagician/Hard Mode Farming/@comment-4813917-20120203213834

Another way (which I find easier) to farm Souls of Light and Souls of Dark is to get some meteorite ore and put about 50 blocks in the Underground Hallow or Underground Corruption. Killing the constantly-spawning meteor heads will give you the Souls. Less set-up time but you can't go afk.