WHERE TO FARM IT
Which area you farm is decided by the area's monster level, not by the item — the rule is the same for everything, so it lives on the mechanics page.
NAMED TARGETS
Named monsters that can be farmed deliberately. The list spans all three difficulties; each monster appears once, at its most rewarding one. The figure is an estimated average kill count.
The Cow King
normal · Lv 28
~1 every 6 kills
The Smith
normal · Lv 10
~1 every 10 kills
Griswold
normal · Lv 5
~1 every 10 kills
Duriel
normal · Lv 22
~1 every 10 kills
Rakanishu
normal · Lv 5
~1 every 13 kills
Treehead WoodFist
normal · Lv 5
~1 every 13 kills