I didn't see that anyone else had this way yet, so I'll post it.
(SPOILER)I was in the Stilshire where you beat Zeromus. Obviously I already killed him. To get to his room, use the teleporter right in the back of the first room.
I had the Warmages Monograph and I was about level 60. If you go back to that room, the skeletons pop back up. I killed all the skeletons, and used the portal 2 times to get back to the room again (be careful becasue the second time you use them it takes you right to where chain killing baddies are). I made 3 trips in there, had my chain at about 70, and it took 10 minutes. The skeletons gave me 99 blood darkened bones, 99 dark crystals, some pebbles and acranas. The bones sell for almost 1500 bucks a pop. All sold totaled about 165000 gil. Not bad for 10 minutes work. In an hour I betcha I could get a million, no problem.