[Skylar]

I have had quite a problem with Greyanna myself and had to load the game 100 times, before i finally found out, what to do.
the best way - for me - was to stand in the room to the right of Greyanna and then opening the door. A lot of searing light and fireballs - and do not pay attention to anyone than Greyanna herself at first - that will be the easiest (haha - there is no "easy" here) way to take her out - followed by some hack and slash from your fighters - block the door with your fighters to get her to come to you.
If you are in the room to the right of Geryanna, there will be a door in the back of the room and behind that - some of her gangmembers. Sometimes they open the door, sometimes they do not. So keep loading the game, at some point, they will not open the door and you can tak them out later.
By the way - make sure that your sorceres and elfs (if you have one) are standing in positions, where it is hard for Greyanna to reach them.
Hope this can help you?