The portal to enter the 70+ dungeon is at 17.0S 61.2W.
All Wisps in the dungeons and on the plateau, and all Pillars in the dungeons count toward the task. The Crystalline Essence Array and its Satellites (which are part of the Crystalline Adventurer quest) also count toward this task.
Return to Oorjit for up to 20,000,000 xp and the Wisp Whipper title.
Speaking to Oorjit after killing 100 Crystalline Wisps:
Oorjit tells you, "Excellent work in your hunt. We'll find your reports very useful."
You've earned 20,000,000 experience.
Oorjit tells you, "Congratulations! You are now truly a Wisp Whipper."
Speaking to Oorjit before the timer is up:
Oorjit tells you, "It's nice to see that you are so anxious to continue the hunt, however I can not issue another hunt at this time. Please return to me at a later time and I may be able to issue another hunt again at that time."
Notes
There are far more wisps in the dungeon than on the plateau. For a mage it would be faster to use ring spells in the dungeons than kill outside on the plateau.
Lower-level archers have an easier time on the plateau.
Lower-level melees can have a much easier time in the dungeons thanks to the walls.