You literally run in a circle killing bombs and old opos. We did this yesterday and actually got the torque on the very last mob we were going to kill. Very epic...but good luck
Go THF first of all if you haven't noticed already. Took me 48 hours of farming and as mentioned above I killed lots of Opo-Opo and Bombs. I made about 2-3 million gil farming it though because of all the geodes and other miscellaneous items I got out of the chests. Good luck!