This should be flagged as a construct, not a monster. The magician/druid Annihilate the Anomalous line works against these, meaning they are constructs.
once in the zone, make a left and soon there is short hill you can climb on your right.
In this area are about 4 feral frightener's mixed in with some other mobs. They roam around and are easy to pull with snare. The cackle spell seems to hit you no matter what, perhaps 2-3 times per pull for me. It probably gets resisted but not evert time. It's only a short time , so if you keep your distance all should be good.
Follow typical root / rot rules... aka, snare, pull to camp, root, and dot away. When you see "a feral frightener begins to cast a spell," sit down as soon as that message pops up. If done correctly, his spell will hit but will not stun you.
Rinse and repeat till dead. This works for the frenzied skeles too.