The thing I do is I spray a repel put a powerful Pokemon in the front of my party then I run/cycle around in the cave near PWT, in the swirling dust spots you will be able to get all four different shards. You may find wild pokémon there too such as Drilbur and Onix.
even though this method is tiresome and boeing, its the best method I have come across.
Hope I helped :D