I hit the motherlode in one type of Mistland dungeon, seemingly a 'hive center'; a large cavern with the red ceiling lanterns, it was a huge complex, had like 5 segments of the resource...filled to the brim with Seeker types, very harrowing experience, but generated a true 'ton' of loot! I've been lucky with cores as well, got 15 in 3 dungeons!