Definitely not weak to piercing. Ranged attacks do 0 damage although extra effects (holy bolts) will proc. Daggers do 0 damage. Still a bash'em up type of mob. Appolyon SE is bizzaro mobs with extreme defense to particular weapons but not as simple as just the opposite of what you would expect. They do link and will agro low HP from quite a distance.
These are very weak mobs all things considered. Located in apollyon southeast, they are weak to what normally would be gimped against them. So that means that they are weak to slashing and piercing type damage. Predator claws consistantly did over 2000 damage which is about 1/3 of their HP. Be careful, they link.
Also their Blood Saber attack drains a LOT of life back... as in put entire alliance into yellow. Keep mages out of range of it and they go down fast.