Suggestion:
For Jin and Amy:
I find it easy to run dungeons in Silver Land: Groove of the Ancients and Ruins of the Silver Knights (GC ph)
I easily got 10-15 fragments per run with NO CHEATS...
For others:
I prefer Marsh of the Oblivion since I easily complete my Sentinel mission in there (GP) in a day with NO CHEATS...
These are MY OPINIONS, don't blame me if it didn't work out the way it worked out on me...