I think this is one of the hardest accomplishments due to the rarity of hendrevails. Best place I've found for "cliff hendrevails" is in the hills above the "hunter's notch" near Nogland in the Vale of Thrain. The spawns here rotate with bears, wildcats, and hendrevails, so just slaughter everything you see and it will bring up about 20% hendrovals when respawning the area.
A hunter's tracking skill can be very handy for this deed. If you know a hunter, give them a donut to help you find hendrevails.