To add to the last comment. NM's pop once every 50-70 minutes around the crystal.
The ones in the lower-tier strong holds are about lvl40. The ones in the stronger holds are in the lvl75 range.
What I found that made getting the 20 shards go more smoother is being in Windurst. If you have outpost warp you can warp to West Sarutabaruta Outpost and run the rest of the way to the crystal in h-8. Then after receiving a crystal, warp back to windurst waters (saving near the zone there) and repeat the process. After four runs, you should wait around for the NM pop to get four crystals before warping back. Which gives you about 8 crystals every hour or so. Best if you have BLM Warp and are part of Windurst already. :)
