Killing black Ceph

I was playing through this building, then I threw a grenade up the stairs, suddenly the black ceph charged on me, I got scared as hell of course, but I managed to kill him!!! then I searched on the internet about it, and read some post saying that you can't kill it at all, then I loaded my save, and did it again to prove then wrong!!! He is very strong, the other time, I had shot him in the head with sniper rifle for about 6 times to kill him!!!