Hello all if you are having difficulty with this quest; like me (that has not healed before and wanted to give it a try); do the following:
1. Keep "Efflorescence" up at all times.
2. Try to stay close to the tank (I know it sounds crazy), but the tank from time to time will pull the adds from you. Also the boss damages the adds somehow.
3. Look at your party frames and keep topping off any of the members (especially the Tank) if you see their health go down. It doesn't matter if it is a little. Use "Rejuvenation" first and then "Healing Touch". Careful when using "Healing Touch", if you use it too much you lose mana very fast. Throw in "Regrowth" and "Swiftmend" from time to time. "Swiftmend" has a 30 sec cooldown so use it on the member at the time that has the least amount of health. It saved me a couple of times.
4. "Cleanse" any member that may have been cursed. I use a neat addon for this called "Decursive". I use it often with my mage, priest and paladin.
5. Continue doing this until you see the boss floating in the sky. Once that happens go back to Lyessa and right click her to get the staff cleansed. Then click on the staff.
6. You last step of the quest is you'll get something to click on your quest bar. Click on it and that will kill the boss and the quest is over.
Good luck.
FYI I usually tank or dps, never heal. It took me many tries to figure out. The really big thing is through the first part of the encounter to keep "Efflorescence" active; I can't stress it enough.
Thank you and again good luck.