Skull Farming - 15 Skulls every 4 mi ...
This is the easiest way I know to farm for skulls. Just select the first level in nightmare and use the windbelt, and you can get 15 cool skulls in about 3.5 minutes. Be mindful of the following:
(1) If you push them onto the train track, be careful if you shoot them because they could land on the other side and get away,
(2) Watch out for Gnolls on the second and fourth waves,
(3) Save your Mana Rage until the final wave
(4) You must have all your Rift points, get "Trapless", and get "untouchable" (don't take any damage) in order to get all 15 skulls.
(5) If you kill them outright (rather than pushing them over the edge), there is a chance that a skull could drop, giving you a total of 16 skulls instead.