I've been wondering what is required to kill the voodoo sisters in the gad temples. Almost all the time when I walk into a room, in for example the prophecy temple, these voodoo sisters seem to be invincible. I've literally shot one for like 15 minutes with the shadowgun + asson without any result (this is when they are already awake of course and firing). Only occasionally they die with just a few shots.
A few dark souls in the prophecy temple are seriously impossible to get, like swinging across a rope with 8 of these voodoo sisters firing away :/.

So, does anybody know what is exactly required for the voodoo sisters to become vulnerable?