You can grind this one fairly quickly every day. Do the Rumble in the Jungle even though this weekly quest may not be available. You can still kill the 3 dudes and I've been getting 6 fragments each day. Then you just need to kill 2 other rare's and your done for the day. I like the Cave of Betrayal at 37/77 because it re-spawns fast. You can also go to the Rainfern Grotto at 17/49 but that does not re-spawn very fast. Please add the cords for your favorite rares.
And then the post below that one:
Came here to say just this, for people still finishing this rep grind. Rendarr, Akrillon, and Eye-Piercer drop 1, 2, and 3 fel crystals, respectively. This is a daily reset (like other rare spawns), which effectively cuts this daily to 4 crystals. Cave of Betrayal is excellent, and you can grab Night Haunter in Zorammarsh and Bramblefell just north of the road there.
As always, if they ever purge their posts, I went ahead and posted the info here as well for future reference.
