Thought I would add a few bits going in to the last area and final boss.
After killing the task mistress and defiler (2nd boss), you move to the final area which is a huge room with lots of wooden platofrms, ramps, mirrors and in the middle is some of the webbing that you need to direct the light to, thus allowing access to the boss.
Getting in your way of doing this is a task mistress who roams the area. Whenever she walks past a mirror, she summons a defiler which in turn, makes the mirror dirty.
So, killling the task mistress before she walks past the mirrors is a priority. The best thing to do is assign the task of grabbing her aggro to one of the group. They can stand in the middle of the room and keep a look out. Meanwhile, move around the room and turn the mirrors/kill the defilers until you open the webbing.
This last boss is now open.
Very simple tactics for a very difficult fight: kill the boss, don't kill the adds (if you want to have any chests to open).
Fortunately, to make things a little easier, the boss and adds come seperately. So, dps the boss and when she disappears, kite the adds around. Rinse and repeat until dead.