Best place to farm Power Crystals?

Sadly I can't remember where I hunted mine, I've got about 45 refined...

Good guess it's either Dual Pirates, Skelegons, or Red Wyverns though. lol.

Skelegons = no. I woulda had my quota filled already if it was skeles.

Dual Ghost Pirates on the other hand... I think I might have NPCed all of those *headdesks*