FIXED as of 13/09/16
The requirement is now reads'' completing any Void Fissure Exterminate mission with an enemy level of 30 or higher and headshots only."
However as thatNoseyParker discovered it doesn't actually work in fissure missions, its needs to be Void missions. He's recommends the node Oxomoco.
The method in Shadow 00 Fox's solution still works for the new requirements.
As of 16/08/16 this achievement is unobtainable!
The changes to the void in the "Spectors of The Rail" update means that T4 Exterminate missions no longer exist.
The dev's are working on a rework to the achievement that will be coming in a later update.
They have no time frame on how long this will take but they are pretty good at fixing such things so it shouldn't take too long.