Over on 360a.org I found two useful bits of information. In my playthrough I had 3 Hyenas grab my leg. The 360a forum posts confirmed my theory on where to go for this achievement.
In the first region, there is a Vista Point south of the hunt in Green Field. It sits just north of the rocks where the river disappears. If you drive to it then stand on top, 2 or 3 packs of Hyenas appear and circle around you. It's fairly easy to farm this achievement here.
I found it was not necessary to stand on the rock. The open field worked just fine. The open field I am referring to is sandwiched between the Vista Point and a Fossil location to the north. To the north by the Fossil is also some Elephant bones. Nearly impossible to miss.
However, as mentioned below, not every Hyena grabs, just the smaller Hyena.
There are 2 size of Hyenas. The smaller ones will grab your leg.
I found this to be correct. The smaller Hyenas have a white stripe on their backs and always grabbed my leg rather than jumped or just bit. I would advise however do not shoot any of the larger Hyenas until you let a smaller grab your leg for fear of scaring them away.
Fairly easy once you know where to go and what you're looking for.