Don't know if things have changed since I have done this quest so bear that in mind.
I had the hardest time finding these guys. And, even when I did find them, it took almost 40 kills before I got the update. There is usually a herd that runs around the griffon fields. I am not certain, but I believe the swarm beetles here are the PHs. With that being said, I have seen the herd in numerous locations. Sometimes they are in the centaur fields and other times they are by the craters. It is quite random.
To make matters worse, they can spawn at a plethora of levels. I have seen them as low as 21 with one or two down arrows and as high as 26 with two up arrows.
It is important to remember, though, that this is part of an HQ and none of the HQs are designed to be easy. All are supposed to be time consuming. If you think this one is long, try the greater lightstone. You will be cursing the Lightbringer wisp in no time flat.