Will to Survive

I'm a squishy mage. She's killed me 12 times in a row now. I just can't seem to hit her hard enough or fast enough to get through the fight.

A small bug note: the quest gives you Will to Survive which doesn't have any cooldown. But if you have a Gladiator's Medallion equiped, then you will get a cooldown

So, I've just managed to kill her with my lvl 51 rogue. The key to the fight is your "Vanish" skill. The strategy is as follows (for rogues only):
1. Let the fight start, go stealth as she can start attacking you, let the NPCs damage her until her health stops going down (takes several seconds, actually).
2. Stay at her side (I stood near Cecilia), deal some damage to get combo points, but immediately stop damage when you are at 40-50% agro (may be you can go a bit higher, I'm not sure).
3. Wait for her to cast stun, Shadow Breath and Vicious Strike. Attack, deal as much damage as you can, but do not spend combo points (you may need them for your Kidney Shot later). Remove stun, run from Shadow Breath and immediately use Kidney Shot when she starts casting Vicious Strike. Use Vanish while she is stunned and go back to Cecilia. You will lose aggro and she will attack the NPCs again (it does not matter if they have any health left).
4. Wait for your Vanish cooldown, repeat steps 2-3.
Took me about 10 minutes. Actually, it can be done by any class having some spell to lose aggro. Do some damage, drop aggro, wait for this spell cooldown, repeat.