I was really lucky on this one.  I'm a level 19 hunter.  I entered the Ziggeraut and hugged the left wall until I could just see the caster to the right on the bottom of the ramp. See him enough to target and get him in line of sight.  I sent my cat after him and he killed the caster.  Masophet then attacked my cat and came up and out a bit to where I was standing.  He was alone and wasn't too difficult to kill.  Like I said... I was just very lucky on that one.