I just spent some time in this zone and I asked one of my friend to help me take out the hivemistress..
I pulled every leech I could see in the first room back to the tunnel where you come from and then soloed them with my bst, and then I took the right tunnel, killed all roaming leeches there, and then pulled the Greater Horror back into the right tunnel, where the hivemistress aggroed, so we had both on us..I kept pet on them, while dru was healing pet and I was slowing and weakening both of those, then the horror aggroed on me, got it rooted, and we simply kept healing my pet and hitting the horror first, then we took on the hivemistress...was not the best way to do it, but still, with calm and patience, it can easily be done.